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Addax Tactical ATAC (Advanced Tactical AR Carbine) GPU

8/7/09 - The Addax ATAC (Advanced Tactical AR Carbine) GPU (Gas Piston Upper) from Addax Tactical is a new gas piston AR upper that utilizes a long stroke piston system, with the piston attached to the bolt carrier rather than the short stroke piston of the earlier PWS system. This was originally developed for the shorty 7" Diablo upper by Primary Weapon Systems (PWS) but is also available for longer barrels.

PWS offers the long stroke system on three different uppers - the diminutive 7" Diablo, the DC-10 with 10" barrel, and the DC-16 with 16" barrel. Addax Tactical is a certified PWS gas system conversion installer with a semi-exclusive on the new piston system, and offers both the short-stroke and long-stroke PWS systems on their GPU builds, which are available in many more configurations. The long-stroke ATAC system is aimed at users who want a virtually maintenance-free upper - almost like having an AK on an AR.

Addax Tactical can still retrofit a customer's existing AR-15 upper with the short-stroke PWS system, but are concentrating their efforts on building and marketing brand new complete GPU uppers. As mentioned in the previous GPU write up, the issue with retrofits is the wide variability of components that the customer might supply. There are so many AR-15 parts manufacturers (some that put out mil-spec parts and some who don't) that retrofitting an out-of-spec or low quality upper may not give the best results. Addax Tactical carries a wide selection of high quality AR-15 components and other 'black rifle' accessories from the major manufacturers, so it makes sense to start with all brand new, high quality components and build all their uppers in-house. While Addax can put together just about any configuration desired by the user, they have standardized models in their series of ATAC GPU uppers that should fit the needs of most customers.

The ATAC is available in a number of different barrel lengths and caliber options, including 6.5 Grendel and 6.8 SPC. Shown here is the mil-spec 5.56mm MK-1 with Sabre Defense 1:7 twist hard-chrome lined 16" M-4 barrel and FSC Bulldog compensator option, mil-spec FA carrier and MP tested bolt with enhanced extractor spring, and mil-spec 7075 T-6 receiver.

Addax ATAC GPU general features and specifications:

  • Operating system - PWS Long-stroke Gas Piston System. All piston parts are finished in Melonite QPQ for wear, heat and corrosion resistance.
  • Available at the time of this writing with choice of 16", 14.5" and 11.5" barrels (and the 7" Diablo).
  • Barrels - Sabre Defense, Daniel Defense cold hammer forged, White Oak Armament.
    Currently chambered in the available calibers: 5.56 NATO (opened up with Ned Christiansen reamer), .223 Wylde (certain models), 6.8 SPC and 6.5 Grendel with M4 feed ramps.
  • Upper receivers - The ATAC GPU comes standard with a Mil-Spec Forged A3 Flat Top Upper Receiver made from forged 7075-T6 aluminum manufactured to Mil. Specs. It includes extended M4 feed ramps, forward assist, dust cover assembly, and the upper is finished per MIL-A-8625. Optional Upper Receivers include the Vltor MUR, with more options currently being looked at.
  • Bolt carrier group - Addax Tactical utilizes mil-spec Bolt Carrier Groups from White Oak Armament, LMT, CMMG and CMT/STAG in their various models. Bolts are MP tested and stamped. Gas rings are left on the bolt.
  • Muzzle device - Addax Bulldog FSC Compensator comes standard, PWS CQB Comp on certain barrel lengths.
  • Magpul XTM rail panels in black for the side and bottom rails.
  • Every Addax GPU Upper is head-spaced, laser bore sighted, test fired (General Function and General Accuracy) and cleaned before shipping.
  • Addax Tactical 3 Year Limited Warranty on all Components, and a Limited Lifetime Warranty on the Gas Piston System parts from PWS

Addax ATAC/PWS Long-stroke Gas Piston System Features:

  • Long stroke piston with two-piece connecting rod
  • The Op-Rod is attached directly to the bolt carrier group to minimize carrier deflection and receiver wear.
  • Hardened tool steel op-rod Carrier key and anti-shear pin
  • Piston and op-rod is completely "mothered"
  • Very easy disassembly for cleaning
  • Virtually maintenance free
  • Non-free float, 4-rail removable hand guard system

I first caught a glimpse of the new PWS long stroke piston system when the Diablo was introduced. "That looks just like an AK", I thought. Instead of the long op rod and separate piston of the PWS short-stroke system, the long-stroke one had the piston as part of the operating rod. When Chris, the owner of Addax Tactical, said that he was going to be utilizing the long-stroke system in his new ATAC uppers, I was, of course, interested to see how they worked. The Addax ATAC GPU Mk2 is shown here, with carbine length gas system.

ATAC Mk2 upper

Addax ATAC GPU details - The ATAC GPU is markedly different from the short-stroke GPU offerings. The most noticeable difference is the proprietary rail/hand guard system. There isn't any 'hand guard', like a conventional free float or non-free float rail system. There are four removable rails that attach at the front to the gas block, and to a trunnion, at the rear. The reason for going with this configuration was to allow the user to replace individual rails in case of damage, and allow servicing by LE or military armourers if necessary. The more open design theoretically allows more airflow for improved cooling.

Gas block - The gas block is a proprietary design, and is pinned to the barrel, instead of secured with set screws like on the short-stroke system. No matter how secure set screws may be, some folks just don't trust them, and will not accept anything less than a pinned gas block. I'll admit that I feel more secure with a pinned one as well. Comparing the long stroke gas block to the short stroke one, the lack of any adjustments, knobs or features is immediately noticeable. It doesn't look much different from a regular DI gas block, only chunkier on top. Instead of a gas tube coming out the back, the piston tube extends back to the receiver, much like an FAL. At the bottom and sides, there are dual threaded stand-offs for the bottom and side rail attachments.

Trunnion - The ATAC does not have a standard barrel nut. Instead, a Vltor VIS-like barrel nut secures the rear trunnion to the upper receiver. The trunnion has 'tongues' on top for the tongue-and-groove attachment for the top rail. The three QD sling swivel cups on the bottom and sides also serve as the rear attach points for the bottom and side rails. The QD swivel cups limit rotation of the swivel to prevent twisting of the sling.

Top rail - The top rail has vent holes on the top and slots cut into the sides, and encloses the operating rod tube, which runs from the gas block into the receiver. The front portion extends past the gas block, for a longer sight radius or more rail space. At the rear, the trunnion has rails on the sides which slide into slots machined inside the top rail. There is virtually no movement between them - it's a tight fit. At the front, the rail is bolted to the gas block with a red-loctited screw. Under normal use and maintenance, there is no need to remove the rails.

Front gas block bottom

Top rail removed

Gas block

Sabre Defense barrel

Side/bottom rails - The side and bottom rails attach to the gas block at the front with two bolts. At the rear, they're secured by the QD sling swivel cups. There is a hole at the rear of each rail, which fits over the sling swivel cup. A spring c-clip/grip ring fits into a groove around the cup and secures the rail in place. C-clip/grip/retaining ring pliers are required for removal. The rear holes in the rail are slightly elongated/slotted, to allow for expansion/contraction of the rails when they heat up. This is done to reduce the influence of rail expansion/contraction on the barrel. The top rail expands along the tongue-and-groove attachment on the trunnion.

Side view

Top and bottom

Long-stroke Piston System - Now we get to the heart of the ATAC; the long-stroke piston system. As I mentioned above, the system is very similar in both looks and operation to the AK system. The gas block has no adjustment. It bleeds gas from the barrel port into the gas block, which expands and pushes the piston to the rear. When the piston head moves past a certain point, gas is bled out through a port at the bottom of the piston tube, above the barrel. The gas tube hole in the front of the upper receiver has been opened up to accept the piston.

Why did PWS come up with this new system? Simplification and ease of maintenance. When looking at the short stroke system, one of the drawbacks is that the front knob and piston would become difficult to remove due to carbon build up. The knob would need a wrench to turn it, and the piston would need to be pushed out of the piston tube from the back by the operating rod. Note that a sticky piston that didn't move freely back and forth under its own weight in the piston tube didn't affect function while shooting. There were also small parts in the short-stroke system.

Piston - The ATAC piston has a beefed up operating rod vs. the short stroke system. It's also a two-piece op rod system. The front assembly is one machined piece, and includes the piston head and what looks like another head at the rear. The rear 'piston' is just a support/guide, which keeps the front piston assembly aligned in the full length piston tube. There's a pin hole in the rear support.

Operating rod - The front piston assembly is attached to the operating rod, which is in turn, attached to the bolt carrier. If you look at the photos, you'll observe that the front piston assembly is allowed to articulate, relative to the op rod, which is why it looks bent at that junction. The rounded-off end of the op rod fits into a hollowed-out section at the rear of the back guide. A tool steel cross pin goes through holes in both parts and holds them together. I was concerned about the durability of that interface, and the connecting pin breaking, but looking closer at the design, the pin is not under any load during operation. The hole in the op rod is oversized so that the op rod bottoms out in the front section when the front piston pushes back on the op rod, and doesn't bear against the pin. During the forward movement of the carrier, the op rod again bottoms out in the piston as it pushed it forward in the piston tube. The piston then bottoms out in the gas block - it doesn't depend on the pin to stop its forward travel. The two parts are never pulled apart in opposite directions (unless the piston head is for some reason, is stuck in the gas block). I asked Chris about the pin and he said that there were no known failures of the pin to date.

The ability for the piston to articulate was designed into the system to prevent any binding of the op rod (as might happen with longer op rods if they flex during the rearward or forward strokes), and also to facilitate ease of installation and removal of the bolt carrier assembly. Also, to allow the piston head to seat properly in the gas block. The charging handle is captured on the op rod but does not have to be removed for routine maintenance. Replacing it is as simple as tapping the connecting pin out to separate the op rod and piston. That's what I did to change the charging handle to the Armament Dynamics ambi one.

The rear of the Operating rod is welded to the carrier key which is made out of hardened tool steel with an anti-shear pin between it and the carrier. The op rod carrier key has been streamlined with all sharp edges smoothed out. There is a center channel between the key screws, which have been staked with a chisel. The displaced metal enters the channel and prevents the screws from loosening. The screws are then further staked with a MOAKS tool on the sides. Addax leaves the gas rings on the bolt. Even though they're not needed to provide a seal between the bolt and inside of the carrier, they provide friction between the bolt and carrier, which reduces the cam pin slamming when the carrier moves backwards. Without the gas rings, the bolt is 'loose' in the carrier and the cam pin can slam against the front and rear of the cam pin slot in the bolt carrier during the cycle.

Carrier assembly removed


Piston close up

Bottom view of upper

Disassembly/Re-assembly - The entire bolt carrier and piston assembly is withdrawn from the rear of the receiver, the same way as on a DI gun. The charging handle assembly remains captured on the rear of the op rod, and does not get in the way of routine maintenance. To re-assemble, the charging handle is held aligned and against the top of the op rod while inserting it back into the upper receiver. The piston head will self-guide into the hole in the front of the receiver and piston tube.

Other components used for this ATAC build

Receiver - The Addax ATAC GPU comes standard with a forged M4 Flat Top Upper Receiver made from forged 7075-T6 aluminum to Mil. Specs. It has extended M4 feed ramps, forward assist, dust cover assembly, and the upper is finished per MIL-A-8625. The ATAC is also available with the Vltor MUR (Modular Upper Receiver).

Barrel and muzzle attachment - This ATAC Mk2 came with a Sabre Defense MIL-B-11595 16" chrome lined barrel with 1:7 twist. Addax reams all their barrels with a Ned Christiansen 5.56 NATO reamer that ensures that the chamber is opened up to a true 5.56 spec (after chroming).
The 5.56 ATACs come standard with the Addax Bulldog FSC Compensator, which PWS makes specially for them. It's essentially a PWS FSC556 which has had the front prongs shortened slightly (hence the name, 'Bulldog'). It has the same compensation of the DNTC/FSC556, with only a slight reduction in flash suppression performance. Besides being a bit shorter, all the sharp edges and corners have been rounded off, giving it a smoother appearance. It bears the Addax logo on the top flat in place of the PWS one.

Misc - I set the ATAC upper up with an Aimpoint Comp M in LaRue Tactical mount, and Troy front and rear BUIS. I replaced the billet PWS charging handle with the Armament Dynamics ACLM ambi charging handle, and installed TangoDown SCAR rail panels and a stubby QD vertical grip.

Initial Range Report - The first time I shot the ATAC was actually when Chris stopped by a range session to drop it off. It was brand new, still shiny with oil. The pics below were taken at the initial range session, before I had set the upper up. Slapped on a T-1 Micro from another upper to give me an approximate aiming point and started shooting. After going through the first mag on slow fire, to allow the brand new parts to 'settle', we then put about 250 rounds through it on rapid fire, slow fire, with most of them double taps. It didn't feel any different than the GPU upper I had - recoil impulse is close to that of a DI gun, and noticeably less than an LWRC or HK 416. With the long-stroke piston and more reciprocating mass moving with the carrier, I expected to observe some increase in muzzle jump or recoil, but I didn't. Neither did anyone else who tried it.
Also on hand was an SBR ATAC with the PWS CQB_556 compensator, which directs all the muzzle blast to the front and was pretty sweet to shoot.

ATAC with T-1


No malfunctions of any kind were experienced with the ATAC at the initial range session. Granted, it wasn't many rounds, but it was a good start. It was hot that day, and I had been shooting my Addax GPU for the first part of the morning. The gas block on the GPU gets very hot after a couple of mags have been put through it, but the free float hand guard doesn't conduct the heat to the shooter's hands as it doesn't contact the gas block. Since the rails on the ATAC are attached to the gas block, heat transfer was one of my concerns. Speaking to Chris, and examining it for myself, the top rail does heat up more than the side rails, as it's attached to the gas block with more surface area contact, and surrounds the gas piston tube. That's why it has the extra holes on top and slits on the side. The side and bottom rails got hot, but not as hot as I expected they would, and not uncomfortably so with rail panels on. Rail panels are recommended, since the gap between them is wide enough to allow a stray finger tip from sliding between them and contacting the barrel. The ATAC comes with the Magpul XTM rail panels that cover the side and bottom rails.

Cleaning - When I got home, I removed the carrier and piston from the upper to see how dirty it got. The piston had a bit of carbon baked on at the front, but that scraped off with a finger nail. All other carbon on the sides of the piston rod easily wiped off the Melonite QPQ finish. The bolt and carrier were practically clean, except for primer residue on the bolt face. No carbon, of course. I ran a bore snake through the bore, wiped everything down, and was done cleaning it. The big difference between the ATAC and the GPU was that the ATAC long stroke piston was much easier to remove, and there were no other parts to clean besides the piston head. It's the closest thing to an AK system for the AR. It is not recommended to stick anything down the piston tube. If something gets stuck, there's no easy way to get it out.

2nd Range report - This time I sat down at the bench to check out the accuracy. I used 55gr Prvi Partizan ammo, but in retrospect I should have brought more accurate (match) ammo instead of the cheap FMJ military stuff. But, my thought was to test it with ammo that it's more likely to be used with. This particular upper was built more as a hard-use carbine than an SPR/precision rifle.
I used a bipod and rear beanbag rest, and a 3-9 power scope. Wind was probably around 5 mph. The results from two 100 yard targets are shown below. I had hoped for a little better, but speaking with Chris afterwards, he's had very good results with better quality ammo, and Prvi Partisan M193 isn't known for its accuracy. On target one, I fired 25 rounds. The max distance between holes is 3.576", which is 3.415 MOA. Average distance to the center of the group (radius) is 1 MOA. Target 2 had 15 rounds on it, for a max of 2.534 MOA and an average distance to center of 0.899 MOA.

Target 1
Target 2

2.5" to 3.5" is definitely acceptable for an M4 with M193. Given what this upper was designed for (reliability and minimal maintenance under heavy use), it'll do the job. When I first looked at the targets, I speculated that the non-free float rails could have something to do with it, but until I try it with better ammo, I can't tell whether it's that, or the ammo. With the four separate rails, there can be differential expansion from heat that can cause stringing as the barrel and rails heat up. The design of the rail attachments are supposed to allow for expansion and contraction, but I'd like to see a free floating hand guard on this system to eliminate that as a factor, and also transfer less heat to the handguard from the hot gas block.

I shot the rest of the ammo (about another 250 rounds) at a variety of 200, 300, and 400 yd steel targets, which weren't hard to hit consistently. No malfunctions of any kind for this session.

Here's a summary of observations/notes:

  • Just like the short stroke piston system, the bolt and carrier stay much cooler than that of a DI gun.
  • The ATAC system functioned with a H buffer and Enidine buffer.
  • The PWS long-stroke system is greatly simplified vs. the short-stroke. Easier disassembly/reassembly, no small parts, no tools needed and less to clean. Easiest AR upper to clean I've ever owned - the bolt, carrier, inside of the receiver all remain well lubed and free from carbon buildup. The front of the piston head is the only place with carbon buildup, and it comes off easily.
  • The piston should be run dry - without lube on it. I lubricated the bolt and carrier the same as on a DI gun - wet with Slip2000 EW lube.
  • Very little sign of carrier tilt in the buffer tube, but the round count is still too low for a long term projection. Minimal finish wear at the rear of the carrier. Normal receiver wear seen at the cam pin nook; same as in a DI gun.
  • The Melonite QPQ finish is very easy to clean (not that there's much to clean anyway). For storage, I put a light film of oil on the piston components and before the next range trip, I'll wipe them dry.
  • No noticeable increase in muzzle jump or recoil over the short-stroke PWS system - feels similar to a DI gun.

The PWS Long-stroke system was intended to be as close to a maintenance-free system as possible, with easier disassembly than the Short-stroke system. Ever since the PWS Short-stroke piston conversion system came out, PWS has been working with dealers and evaluators like Addax Tactical to refine and improve the system, which led to the Long-stroke system on the Diablo, which was adapted to longer platforms. I think that the Long-stroke system is an improvement over the earlier Short Stroke system, because of the ease in which it's disassembled, the fewer parts, and practically nothing to clean except for the piston head. Every piston conversion has had their share of technical challenges, and only time will tell how they fare.

I don't really think that reliability of the DI (Direct Impingement) system is in question nowadays, from those who are familiar with it. ARs will run when dirty but well lubricated, but can be a messy affair to keep a dirty gun running. If you prefer a cleaner-running system, and are looking for something almost maintenance-free that you're going to shoot a lot and often, then the ATAC might be a good choice, if it proves itself to be robust and reliable in the long run.


Addax Tactical ATAC GPU Upgrades and FRS Anti-Carrier Tilt buffer

2/4/10 - Addax Tactical and Primary Weapon Systems (PWS) continually work on refining their gas piston upper systems, and when Addax informed me of some available upgrades to the Addax ATAC (Advanced Tactical AR Carbine) GPU (Gas Piston Upper) which I featured previously, I was interested. As covered in the writeup above, the ATAC GPU is a gas piston AR upper that utilizes a long stroke piston system, with the piston attached to the bolt carrier rather than the short stroke piston of the earlier PWS system. This was originally developed for the shorty 7" Diablo upper by Primary Weapon Systems (PWS) but is also available for longer barrels. Please read the above writeup for working details of the ATAC GPU.

The MK-1 ATAC featured above had a Sabre Defense 1:7 twist hard-chrome lined 16" M-4 barrel and FSC Bulldog compensator option, mil-spec FA carrier and MP tested bolt with enhanced extractor spring, and mil-spec 7075 T-6 receiver. I sent it back to Addax for the following upgrades:

  • Vltor MUR upper receiver
  • Daniel Defense Cold Hammer Forged (CHF) mil-spec 16" carbine 1:7 twist chrome-lined M4 profile barrel
  • Microslicked-coated mil-spec FA bolt carrier group
  • PWS piston bolt with spring
  • Armadynamics Ambi charging handle

Vltor MUR 1A upper receiver - The Vltor MUR (Modular Upper Receiver) is probably my favourite upper, mainly because of the location of the forward assist assembly. The 1A shown here is the bolt Assist model) while the 1S only has a shell deflector; no bolt assist. The 1A that Addax installed on my ATAC has the 2nd gen forward assist assembly, which is less 'blocky' than the original design. The first MURs were machined from billet, but are now all machined from forgings. The forwards assist plunger is located further forward than a standard forward assist, which gets it out of the way of the receiver sling mount for left-handers like me, and also provides more clearance for my finger when using an ambidextrous charging handle.

Daniel Defense Cold Hammer Forged Barrel - The DD CHF barrel is manufactured completely in-house at Daniel Defense, and is constructed of 4150 Ordnance Grade Steel, and MPI and HP tested. It's chambered in 5.56 NATO with a 1:7 twist rate. The bore and chamber are chrome lined. The barrel features an A4 barrel extension (M4 feed ramps), and is threaded 1/2x28. The exterior is heavy phosphate coated to military specs. DD CHF barrels have the month/year of manufacture stamped on the barrel. The M4 feed ramps line up perfectly with the ramps on the MUR receiver.

Vltor MUR

Gen 2 fwd assist

DD CHF barrrel, Bulldog comp

Microslick coating on bolt carrier - The Addax ATAC bolt carrier is now coated in NIC Industries (Cerakote devision) Microslick (ceramic dry film lubricant). It's a dry film lubricant that withstands high temperatures and pressures, and provides lubrication, wear resistance, and corrosion resistance. The coated bolt carrier is a dark grey.

Armadynamics Ambi Charging Handle - Being a lefty, I had previously swapped out the standard charging handle on the ATAC with the Armadynamics ACLM ambi charging handle. Addax now offers it as an option. I have been using them on most of my uppers now. Combined with the Vltor MUR, it's the best set up for lefties I've found to date.

PWS Piston bolt with spring - The new PWS 5.56 AR Piston Bolt is heat treated to mil spec and MP tested. The bolt has no gas ring groove, but instead has a modified shoulder that is designed to accept a spring behind the bolt. The spring provides a slight amount of pressure that ensures that the bolt is in its most forward position in the receiver extension when in battery. One of the reasons why I left the gas rings on the bolts of my piston guns was to provide some drag inside the carrier, to help reduce the cam pin slamming into the carrier. During the cycle, a bolt without gas rings is free to 'bounce' back and forth a minute amount, and the cam pin will slam around in its channel and at the end of its movement.
When in battery, there is still a minute bit of slop in the bolt, and at the time of firing, it may not be in its most forward position. On a DI system, the gas expands into the space between the tail of the bolt and the bolt carrier, and pushes the bolt and carrier apart, ensuring that the bolt is pushed forward into the receiver extension. On a piston gun, the bolt is just unlocked by the backward motion of the carrier when the piston/op rod moves back. If the bolt isn't in its most forward position, the cam pin can make premature contact with the cam pin recess on the inside of the receiver, causing more wear than with a DI gun. The wear might be self-limiting and not an issue, but it also implies more stress and wear on the components than if it was prevented or reduced. The PWS piston bolt with spring is designed to reduce this by keeping spring pressure on the bolt so the slop is taken up. The bolt is for use in piston rifles only, and will not work in DI rifles.

Carrier, charging handle and Op rod assy

Microslick'd carrier

PWS piston bolt

When I got the upper back from Addax, I installed the same components as I had on it before - the Troy Ind/Noveske sights and the Insight Tech-Gear MRDS optic.

M4 ramps

Firearm Ready Solutions, LTD Anti-Carrier Tilt (ACT) Buffer Assembly

Due to the offset force acting on the bolt carrier, piston rifles in general experience 'carrier tilt'. Whether it is by means of an operating rod fixed to the carrier, as in the ATAC, or an operating rod acting on a carrier key, the force driving the carrier rearward is still offset from the centerline of the carrier. Because of this offset, the carrier has the tendency to tilt upward in the front, pivoting about its bottom rear edge. The bottom rear edge of the carrier usually wears on the inside bottom front of the buffer tube, right behind the buffer retaining pin. Some piston uppers exhibit more wear, others less. Other signs of carrier tilt is more finish wear on the upper bearing surfaces of the carrier, on each side of the carrier key near the front, than on the lower bearing surfaces.

Shown below is a photo of the buffer tube that I've used for all my piston (and much of my DI) rifle shooting, and it's been through a few thousand rounds with three different piston uppers. The anodizing has been worn through where the bottom of the carrier begins its rearward movement into the buffer tube. The wear is quite minimal though, and is little more than the anodizing being worn through.

While wear like this isn't enough to cause me concern, I've seen deeper wear in other piston rifles, and it's still an issue to deal with in the overall AR piston design. Most designers/manufacturers of AR piston rifles have come up with their own fixes to the phenomenon, usually with modifications of the rear of the carrier (skids, larger ends, enlarged bearing surfaces etc).

A member of M4Carbine.net, Seth Harness, worked on a solution to the carrier tilt issue in piston ARs. His solution was to make a buffer with an indexing nipple on the face of the buffer assembly that fits into the hole in the rear of the carrier. The buffer assembly holds the rear of the bolt carrier centered in the buffer tube (receiver extension) during the linear motions of recoil and battery, eliminating the wear associated with carrier tilt. Seth started a company, Firearm Ready Solutions, LTD, and is now offering the Anti-tilt Buffer assembly as a drop-in cure for carrier tilt in the piston operated AR15/M16 platform. It was great timing that the ACT buffer became available just around the same time I was going to get my upgraded ATAC upper back, so I could try them both together. Shown below is the ACT buffer and special spring assembly.

Anti-carrier tilt buffer - The ACT buffer is machined out of 7075 T6 aluminum and Type 3 hard coat anodized without dye (clear) per MIL-A-8625F. The hole at the rear of the carrier varies in size between manufacturers, as it is not a critical dimension, so the customer has to measure the hole accurately, and each buffer is made to order. It is important that the hole is measured with calipers at least .050" inside the rear of the carrier, not with a ruler as most holes have a chamfered edge which is not the right measurement.

The ACT buffer approximately the same overall length as a standard carbine buffer but has a much longer bearing surface (the larger diameter) than a standard buffer. A standard buffer is subjected only to a forward-aft force with little if no lateral force. With a piston gun, during the initial movement of the carrier, a downward force is applied to the nipple on the head of the buffer by the rear of the carrier trying to tilt down. The rear of the carrier is prevented from contacting the bottom of the buffer tube as it is held by the nipple. The nipple, in turn, transfers that force to the body of the ACT buffer, which bears along the bottom of the buffer tube initlally. The larger surface area spreads that load over a much larger area, and is also aluminum instead of steel, which reduces the wear on the inside of the buffer tube. Once past the initial starting movement, the buffer is centered in the tube and continues to move rearward. The carrier is kept centered at all times during its movement.

The body of the ACT buffer is also a larger diameter than the standard buffer. This allows the buffer spring to provide support to the body itself. With the standard buffer and spring, the buffer body is much smaller than the inner diameter of the spring, and is free to wiggle if any side load is applied. Since the ACT buffer has to encounter side loads which may otherwise tip the end, the extra support from the spring prevents it from tipping inside the tube.

The ACT buffer is hollow, and filled with custom cut tungsten and steel weight combinations to make up CAR, H, H2, H3, 9mm and A2 weights, depending on what the customer specifies. High grade EPDM padding is used between the weights. The A2 (rifle length) buffer is in the testing phase at the time of writing. I requested a H2 weight, which seems to work well for all ammunition types in my ATAC upper. The buffer is closed on the end with a rounded blue plug made by Blackjack brand. The rounded design is supposed to slow the velocity and impact of the buffer assembly gradually, vs. the flat contact end on the end of the standard buffer.

Flat Spring - The ACT buffer is designed to be used, and is supplied with a Superior Shooting Systems chrome silicon flat wire buffer spring. It is not recommended that a standard round spring be used with the ACT buffer. This flat wire spring has a flat cross section rather than the round cross section of standard round wire springs. The design of the flat wire spring operates with lower stresses than similar round wire springs, which improves the consistency of the spring throughout its working life. The CS flat wire spring takes up less room than round wire (as it's flat), which means that there is more space for the spring to function. The CS flat wire buffer spring is longer than a standard carbine spring (see the photo below) but when compressed, takes up less room than the round spring. At the installed length, the CS flat wire spring produces about 20% greater power than a stock carbine spring with a lower spring rate than a standard round spring. This results in delayed unlocking and a smoother return to battery for the bolt carrier. The flat wire spring can be 'tuned' for lighter loads by cutting off one coil at a time if necessary. The flat wire spring also works with standard buffers.

ACT Buffer

ACT buffer (top), carbine buffer (bottom)

CS flat wire spring (bottom)

Round (top) vs. flat spring

Installation - To install the ACT buffer and flat wire spring, the standard buffer and spring are first removed. A bit of grease is put on the inside mouth of the buffer tube/receiver extension which helps the flat wire spring slide in easier. The spring is put in first without the buffer. While holding the spring in, the buffer is then inserted, with the ramp machined into one of the three flats helping to push the buffer retaining pin down. I put a light coat of grease on the buffer before installing it.

Installing the upper to the lower can be done by either placing the upper so the rear of the carrier is just slightly forward of the nipple, then pushing the upper rearward into place and then pushing both the front pivot pin and rear takedown pin in. Or, the upper can be attached at the front pivot pin first, then swung down until the bottom rear of the carrier is sitting on top of the nipple. Using a finger to slightly depress the buffer, the upper receiver with drop into place.

To remove the upper from the lower, both the front pivot pin and rear takedown pins are pushed out and the upper and lower separated. The upper cannot just be pivoted open by pushing out the rear takedown pin only since the rear of the carrier is captured by the buffer nipple.

Buffer and spring

Buffer nipple and carrier hole

How they fit together

ACT buffer installed

'Newish' tube

When cycling the carrier by pulling back on the charging handle, I found that it felt smoother than a standard buffer and spring. To test out the ACT buffer, I replaced the receiver extension with another one that had not been used with a piston system, so it was free from carrier tilt wear.

Initial Range Report - Well, not really the range but the desert. It was chilly and windy, so this was more of a break-in/function test than anything else. I did a quick zero of the MRDS and irons on paper then proceeded to put rounds through the rifle. It was too windy to do any kind of accuracy testing as the wind was making the cardboard target move. Shooting then commenced at steel plates at about 30 yards and 100+ yards. I put about 200 rounds of Wolf 55gr though it and 100 rounds of M855. Not much, but that's all the time I had. No malfunctions of any kind were experienced.

I was curious to see whether the CS flat wire buffer spring would be too long or strong with the weaker Wolf ammo, but that didn't prove to be the case initially. For some reason, at a later date, I did have some failures to lock back, so I trimmed a couple of coils off the CS flat wire spring. No issues since. The carrier locked back after the last round with TD ARC, USGI and Pmags. One thing I noticed was that there was no spring sound from the buffer tube when shooting it. It never bothers me anyway, but some folks do notice it. As I mentioned above, cycling the charging handle feels very smooth, and so does operation. Felt recoil/impulse is still the mildest of any piston AR upper I've shot.

300 rounds isn't much, but is normally enough to see small signs of wearing in, in the usual places like the bottom rear of the carrier in a piston system, or in the receiver at the cam pin recess. I checked the upper and lower when I got home and didn't see any new marks inside the buffer tube or on the carrier, and only a slight amount of surface wear at the cam pin recess, which is normal in all uppers. Everything looks promising so far, so I'll update this writeup when I've put more rounds through it.

Update 2/21/10 - Put another 350 rounds through the upper a week ago and I see no additional signs of wear. No carrier tilt wear in the buffer tube.

Update 6/5/10 - Just went past the 1000 round mark, and no signs of carrier tilt.

Update 5/14/11 - Approximately 3000 rounds through the same receiver/upper combo, and only very slight wear on the anodizing is noticeable inside the buffer tube. No gouging of the material at all. Went through a 2-day advanced tactical carbine class with this combo without a single malfunction, where we ran our rifles pretty hard.

Firearm Ready Solutions Anti-Carrier Tilt (ACT) A5 Buffer Assembly

7/17/11 - Firearm Ready Solutions (FRS) has come out with an A5-length Anti-Carrier Tilt buffer assembly, to work with the Vltor Weapons Systems A5 System.

The Vltor A5 package was originally designed as a replacement stock system for users of the the M16A4 rifle (as found on the M16A4) who want to maintain the same reliability, carrier velocity and rate of fire as the rifle-length A2 stock, while adding the functionality of an adjustable stock. This is achieved by a new buffer and receiver extension (buffer tube), and the use of the standard rifle length buffer spring. Since it was released, the A5 has been installed on many M4 systems, to include SBRs, carbine and mid-length gas systems. It's gaining popularity as some have found increased reliability by simply switching over to the A5 system, and a smoother cycle.

The A5 uses a longer buffer tube and receiver extension. The A5 buffer is approximately the same length as a rifle buffer with the 'spacer' portion at the front removed. It is 4", versus the 3.25" of a carbine buffer. The Vltor A5 receiver extension is 3/4" longer, making it 8" long, vs. 7.25" for a carbine tube.

FRS contacted me and asked if I were interested in trying out an A5-length version of their ACT buffer. Since I was already using the A5 system on one lower, I said 'you bet'. When using the A5 system, some folks 'tune' the buffer weight to the ammo they shoot the most, by replacing the heavier tungsten weights with the lighter steel ones. The A5 buffer comes with 2 steel and 2 tungsten weights. For one particular upper that seemed to have trouble cycling lighter loads (on both carbine and A5 systems), I replaced one tungsten weight with a steel one (so it has 3 steel and 1 tungsten), and that did the trick. I call it the '-1' weight, since it's a bit lighter than the A5 weight. That combination comes out to about 4.5 oz. So, I requested that FRS make the A5 ACT buffer around the same weight. FRS uses custom cut steel and tunsgten weights, and can make the buffer to match customer requirements/specifications. FRS made it 128 grams, which is almost exactly 4.5 oz, and stamped the weight on the head (128G) for reference. The nipple size (inside diameter of bolt carrier) is stamped on one of the flats (.625"). It's useful to have that info on the buffer.

Carbine vs. A5 length ACT buffers

A5 (top) and carbine below

A5-length ACT buffer

The only external difference between the A5 buffer and the carbine ACT buffers is the length. For more details on the FRS ACT, please read the writeup immediately above. The anodizing on the A5 ACT was also a bit darker. The above photos show the difference between the two. As mentioned in the initial ACT writeup, the ACT requires the use of the Superior Shooting Systems CS flat wire spring, which it comes with. For the carbine length system, I ended up cutting a couple of coils off the spring, as it's pretty stiff and I was shooting lower powered ammunition through this setup. For the A5 system, I left it the stock length, and didn't cut off any coils. My Addax Tactical ATAC GPU is my most-used upper, as it's proven very reliable in combination with the carbine-length FRS ACT, and it's easy to clean and maintain without having to keep it soaking wet in lube. A bit of Frog lube (or whatever) is all that I put on it.

I now have about 900 trouble-free rounds through the through the Addax ATAC GPU with the A5 length ACT, using a mixture of Wolf, Prvi 55 gr, M193 and M855. About 500 rounds of that in a one-day carbine class. What can I say - it was pretty boring - it all worked. All loads locked the carrier back on the last round, and I didn't experience any malfunctions. In general, I've found the A5 system-equipped lowers to be smoother shooting than the carbine ones. I'll keep using this combination, and report any problems if there are any. But so far, so good. If you want to run the A5 system with a piston upper and FRS's ACT, you now can.




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