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3/15/13 - The BCMGunfighter Compensator from Bravo Company USA (BCM) is a compensator for the AR-15 platform (or any rifle with the same barrel threads) designed specifically for tactical applications. The BCMGunfighter's intention is to reduce muzzle rise, flash signature, noise and concussion when compared to other typical compensators on the market.
As discussed in previous writeups, there has always been a market for muzzle devices designed for the AR-15 ever since I got my first one in the late 80's. The standard A2 compensator is still the most economical choice, but there are always folk looking for improved performance over the standard. Flash hiders reduce flash signature while compensators or brakes either aim to reduce felt recoil or muzzle jump/movement. The idea is to keep the muzzle as steady as possible when shooting, so that any movement seen by the shooter is due to his own input. This translates into quicker follow-up shots, or less need to adjust one's position after firing a shot. While brakes have always been available to the 3-gun/competition crowd, 'tactical comps' are a slightly more recent thing. I think that their popularity is due in part to the proliferation of tactical training over the past decade, and the timed drills that require fast, accurate shooting. Granted, the 5.56mm round doesn't have much recoil to begin with, but if you could make it shoot like a .22 LR, why not? For the most part, muzzle brakes do a decent job of reducing muzzle movement but at the expense of increased side blast, flash and concussion. Then there are muzzle devices that attempt to strike a balance between all the desired attributes, like the PWS FSC556, Battlecomp and Griffin muzzle devices. The BCMGunfighter Mod 1 Compensator's release was much anticipated by those who use comps (like me); wondering whether this would finally be the magical muzzle device that does it all (suppressors excluded of course).
Description - The BCMGunfighter Compensator is currently available in two models at the time of release; the Mod 0 and the Mod 1 (illustrated here). The Mod 0 is a shorter A2-length device while the Mod 1 is longer, extending the length of a 14.5" barrel to meet the 16" minimum requirement when permanently installed. The Mod 1 is 2.15" long, and I measured the diameter at the front and rear 'rings' to be .860-.861". It's made out of stainless steel with a black melonite finish.
Starting from the rear, the Mod 1 has a 'standoff', with the wrench flats forward of this standoff rather than at the very rear. This standoff allows wider wrenches to be used on the wrench flats without interfering with crush washers or peel washers. A crush washer is supplied with the Mod 1. The Mod 1 is compatible with all suppressors that attach to the standard GI comp/flash hider, and most blank firing devices. The portion forward of the standoff provides a controlled interface for suppressors like the HALO to engage, which key off the rear of the comp. When suppressors like the HALO are installed on GI hiders that have crush washers, the crush washers do not provide a square/aligned interface, and can cause the suppressor to angle off when tighened, ever so slightly. The Mod 1 prevents that from happening, and ensures that hider-mounted suppressors are aligned with the bore. At the bottom of the standoff is a pre-drilled hole for permantely pinning and welding the comp to a 14.5" barrel for legality. A pin is supplied in the packaging.
Moving forward, we now come to the 'cage' of the Mod 1. The Mod 1 has six slotted ports, with the side ones being longer than the upper and lower slots. The upper and lower slots are equal in length, but the lower slots are wider. There are no slots on the top or bottom of the Mod 1. The side slots are located slightly higher than 3 and 9 o'clock. Visible at the rear of the long side slot is the first expansion chamber. This chamber is where the gases enter when they first exit the muzzle. There is a small port on each side that vectors the gases slightly up and forward. The first expansion chamber also adds to back pressure which is intended to increase reliability in some setups. The shorter Mod 0 comp does not have this first expansion chamber.
Forward of that is the main 'blast chamber', or cage. The Mod 1 comp is actually a two-piece design. The front 'end cap' is permanentely attached to the cage and incorporates an internal truncated cone. Some of the gas that exits the 1st expansion chamber through the center hole are directed down the sides of this cone and the rest expands out of the slots. The gases coming out to the side 'cancel out' the 1st gas streams. Early prototypes of the Mod 1 had a non-truncated cone with a sharp edge. This sharp edge would erode and mushroom over from muzzle blast, and after a certain number of rounds, the erosion would stop. The next prototypes were made with truncated cone shapes that mimiced the eroded shape, with a blunt edge rather than a sharp edge. It's 'pre-eroded', in other words and additional erosion is stopped/ minimized past the initial few hundred rounds.
The main reason that the Mod 1 is an 'open' design vs. a 'closed' design like the KAC Triple Tap, Battle Comp, or Griffin Flash Comp/M4SD is to provide more flash consistency. Closed comps trap particles in their 'burn chamber' consisting mostly of unburnt powder and maybe some copper jacket flux, that build up and randomly ignite, causing a large flash. I've experienced this phenomenon with all the closed comps I've used. I'll be shooting, and there'll be minimal flash, then suddenly a larger fireball, then back to normal. As a civilian shooter that doesn't shoot much at night, it's not a big deal for me.
The end cap has a star shaped exit hole - this is actually more cosmetic than functional, and provides a unique appearance (looking like the BCM 'star' logo) from the front. The raised 'rings' at the front and rear of the comp are of the standard A2 compensator diameter and provide the bearing surfaces for suppressor attachment.
The BCMGunfighter Mod 1 comp can be installed with the provided crush washer to achieve the correct clocking so that the closed section is on the bottom, but peel washers can be used as well. The comp does need to be clocked, and I recommend installing it without any bias to either side (keep it neutral instead of biasing it to one side to compensate for grip/hold from a particular shoulder).
Notes/Observations - First of all, I'm not able to scientifically 'test' muzzle device effectiveness, so all comments here are based on my own subjective observations, as well as other shooters that were with me. At the range, I brought along the PWS FSC556, the Battlecomp (BCE) 1.0 and the Griffin Armament Flash Comp to swap out with the BCMGunfighter Mod 1 for comparison. I had previously done a writeup on Griffin Armament's muzzle devices, which included other muzzle devices including the Flash Comp and shot some comparison videos of them. Swapping out comps quickly on the same weapon using the same ammo is the only way I can compare them - as different barrels or setups would perform differently. Out of my previous testing, I found the Flash Comp to be the most effective hybrid comp, as far as muzzle control goes. That's the standard that I wanted to compare the BCM Mod 1 to.
In the video, I wanted to limit the amount of input that I imparted to the weapon, so I only supported the handguard from the bottom; cupping my hand slightly and not pulling it back into my shoulder. The second part of the video shows more controlled fire. Here's the video that includes the BCM Mod 1 along with the other comps. A friend was interested in trying out the Flash Comp which is why there's more footage of it than the others.
Muzzle movement - The general consensus was that all four comps were pretty effective at controlling the muzzle, and we had to shoot them and swap them out repeatedly to compare them. The FSC556 and BCE 1.0 work well, with the FSC556 having more side blast than the BCE. The BCE 1.0, as mentioned before, pushes the muzzle down slightly. The BCM Mod 1 is very effective at reducing muzzle jump, and any movement tends to be more horizontal than vertical. The muzzle stays flat, with a bit of side to side movement. The Flash Comp is very neutral overall, and movement is about equal vertically and horizontally. The red dot 'floats' around a small area.
Felt recoil - When it came to felt recoil, the shooters agreed that BCM Mod 1 felt like it had a sharper recoil impulse with a bit more felt recoil than the BCE 1.0 and Flash Comp. The reason for this, I'm surmising, is because the gases exit the Mod 1 to the sides and also forward, following the angle of the cone in part, whereas the gases from BCE and Flash Comp devices exit at 90° to the centerline.
Side blast and concussion - All the comps were unpleasant to stand right next to (within three feet). One thing I've noticed is that open 'hybrid' comps like these are not as concussive off to the side as pure muzzle brakes like the Surefire, or Griffin Muzzle Brake, but they seem to have a bit more blast than the closed comps. I'm not sure whether it's due to the volume of gas coming out the larger open designs vs. the smaller and quicker jets coming out the small holes or slots, but that's just my subjective observation. I also think that since the BCM Mod 1 vectors some of the gases forward, you don't feel as much standing directly to the side as other open comps. I didn't stand forward of the comp during shooting.
The one thing I do find with open comp designs is that I get more particles hitting me in the gas stream than with closed designs when standing to the side. The particles can include unburnt powder, primer residue, copper jacket flux etc. Due to its conical baffle, the BCM Mod 1 does direct particles to the side and forward - more so than the BCE and Flash Comp. The reason that BCM went with an open design was more for consistent flash reduction. The small ports or holes on the closed comps trap the particles better, but then they start building up then combust, causing a larger than normal flash. A good illustration of that is in the video when I'm shooting the Flash Comp. When I start rapid firing, the 24th shot has a large flash whereas all the others are small or unnoticeable. The BCE does this as well. The open comps tend to have a more consistent flash, whether it's large or small, because they expel the unburnt particles more completely.
Particles exiting any comp can be a concern in a tactical environment, so it's important to note that the BCM comp is really designed to be used on barrel lengths 14.5" or longer. With the right ammunition and barrel configuration, the only particles that will be coming from the BCM comp will be unburnt powder. If the user uses crappy ammo, and/or the wrong bullet weight and rifle twist, you may get copper flux particles, but very little. With the new powders that the military is testing and adopting, this will be a non-issue. The new powders are getting full burn within the first 8 inches of barrel length.
There are advantages and disadvantages to both type of comp designs (open vs. closed). The BCM offers more consistent flash, but possibly increased unburnt particles coming out the sides. With enclosed systems you can have carbon, copper flux and unburnt powder build-up within the comp. The downside to that is the sporadic flash and possible unsafe dislodging of copper flux and carbon, which may damage the comp or act as shrapnel to the person beside you. Smaller ports can also clog up eventually reducing the effectiveness of the device and requiring cleaning, whereas opens designs are less prone to clogging up.
Flash suppression - Unfortunately, I couldn't stay at the range past dusk, so I wasn't able to do any night time testing of the BCM Mod 1. If you search on youtube, you'll find a couple of comparison videos showing the BCM Mod 1 at night, where it looks like the flash is comparable to the BCE on a 14.5" or longer barrel. It's not recommended for SBRs (no hybrid comps are, if flash suppression is an issue), and will exhibit the same large flash as other comps on them.
Bottom line/conclusions - When it comes to comparing hybrid comps that are designed to find that balance between a flash hider and muzzle brake; of recoil and muzzle rise reduction, concussion and flash suppresion, the latest ones are all very close in performance; each better than the other in one respect or another, but none better in all respects. The Mod 1 is no different in that respect, and it does a very good job of providing muzzle control. From what I've seen of the flash from other videos, it does produce more consistent flash than closed comp designs. The downside to the more consistent flash is increased particulate in the gas streams due to the open design, which may be a consideration when shooting in close proximity to others in certain environments. It's not an issue on the square range when everyone is a few feet apart and shooting from the same firing line, but may need to be evaluated if it will be used when a team member is right beside or forward and to the side of the muzzle. As mentioned before, the use of ammunition with powders that burn more completely should address that concern, as well as the proper twist rate and bullet weight combinations.
I think that it'd be a good idea for muzzle device manufacturers to state the parameters under which their devices are designed to work within, or what the consumer should expect optimal performance with. Someone using military ammunition on a carbine might find that a device works as advertised whereas some guy installing it on his 7" AR pistol might think it blows when shooting bi-metal Russin ammo through it.
The BCMGunfighter Mod 1 is a really good hybrid comp - as are the other comps I mentioned, but they're not magical; and based on the laws of physics, that's sort of to be expected. Each has its pros and cons which the end user has to weigh based on his own needs. I've spoken to various muzzle device manufacturers about finding that 'magical pill' that does everything well and is better than all the competitor's products in all respects. All of them agree that it's a real challenge, as improving one aspect usually affects another one adversely. If noise, flash and concussion are indeed extremely important considerations in a tactical enviroment, the end user really needs a suppressor. However, suppressors may not be an option or available for many operating in a tactical enviroment other than SWAT teams or SOF units. It's becoming more evident to me that it just boils down to which parameters are more important to the end user, and the end user will have to choose their device depending on their priorities. But I'll continue to applaud manufacturers who try to bring us better products, and accept the challenge to achieve something that does evreything better. It's a win-win for the consumer.
Rainier Arms Raptor™ Ambidextrous Charging Handle by AXTS
10/27/12 - The Raptor is an ambidextrous charging handle for the AR15/M16 series of weapons, made for Rainier Arms by AXTS Weapons.
Background - Back in August of 2011, Josh at AXTS showed me some CAD drawings of an ambidextrous charging handle concept that he had been working on. As the resident ambi-advocate, I immediately jumped at the chance to provide my input, as I had on the AX 556 lower receiver. As a long-time advocate for ambi controls on ARs, I pretty much have what I've been waited over 25 years for - properly designed ambi selectors (Battle Arms Development), and a fully ambi lower (AXTS). There have been other ambi charging handles on the market for a while, like the Badger or ACLM, but none that I had input with from the start. One of Josh's main concerns was whether there was a market for another ambi charging handle, and I replied that I thought there was, especially if it offered something different from the others.
Over the next few months, Josh worked on the design; sending my updated drawings while I provided my .02 worth. In February 2012, he came to the point where he was ready to make a prototype. The first few prototypes had three different size handles, to test which ones would be optimal for use. There was a standard (short), medium and long handle. From the input of myself and other testors, Josh chose the lengths to put into production. AXTS partnered up with Rainier Arms on this project; AXTS designed and manufactures the Raptor for Rainier. The production version eventually became the Rainier Arms Raptor that you see here. The Raptor can be purchased through Rainier Arms as well as directly through AXTS.
Description - The standard charging handle design has a shaft with an integrated T-handle, with the latch fitting into a slot on the left side. Most aftermarket ambidextrous charging handles are variations of the same configuration, except for the ACLM, which has a completely separate 'T' from the shaft. The Raptor's shaft and T are one piece, but the head of the T is much smaller. Instead of the head of the T housing the latches, the latches make up the T. They both pivot back and forth on the downsized body. You can see an animation of it here.
The pin diameter is greatly increased over that of the standard roll pin, and a coiled stainless steel spring pin is used instead of the standard roll pin. The standard 1/16" roll pin used in the USGI charging handle has a double shear strength of 457 lbs. The 3/32" 416 SS Spirol pin used in the the Raptor is over 2.4 times stronger with a double shear strength of 1150 lbs. When extended latches are used with standard charging handles, the twisting imparted to the charging handle slot is quite often what causes the failures of standard charging handles. The slot containing the latch begins to spread open causing the already small diameter roll pin to lose its hold in what had been an already thin wall section of material retaining it. On the Raptor, not only are the pins far stronger but they are pressed into a much thicker area of material than a standard charging handle, and with the straight back pull on the latches/handles, there is virtually zero torquing of the latch mechanism pivots.
Rather than restrict themselves to the same envelope as the standard charging handle, AXTS designed the Raptor for effective manipulation of the charging handle using the pinch or blade (palm) techniques. They had to find the right balance between ease of manipulation and size. This was part of the prototype testing process with the different sized handles. The Raptor has left and right latches assembled onto the shaft. The left one is slightly shorter than the right one. This is because the right side of the upper receiver has the forward assist, which can get in the way. It does not extend past the forward assist. The left handle also incorporates the hook, which engages the notch in the receiver. Both handle have a forward angle to them, which helps keep the fingers from slipping off. A good grip on the charging handle allows the user to better control the pull, and prevent the shaft from being pushed or pulled to one side. The result is a smooth, straight-back, direct pull. The latches were also designed to elminate any pinch points that might pinch skin or gloves when actuating the charging handle.
The shaft of the Raptor is made of 7075 aluminum which is hard anodized, and the handles/latches are made of steel. Steel instead of aluminum was chosen to provide strength when clearing malfunctions, and also because of the concern for wear at the camming surface between them. They've been skeletonized for weight reduction. Future testing may be done on aluminum handles to see whether they can perform as well as the steel ones for additional weight savings. The handles have vertical grooves on the front and back surfaces for traction.
The Raptor is shown below, placed on top of the BCM Mod 4 and PRI Gas Buster charging handles, just for comparison.
The issue that users of ambi charging handles can encounter when used with a standard upper receiver is apparent in the photos below. The forward assist on the right side of the receiver is located such that it can interfere with the operating hand. A 'pinch grip' is necessary when operating the charging handle on the right side, instead of a palm blade technique, as there's no room to accommodate a hand there. The length of the Raptor's right handle is extended so that a more positive grip can be obtained on the right side, even with the forward assist there. It's easier to manipulate than other ambi charging handles with standard sized latches/bodies.
If a forward assist is desired, the Vltor MUR 1A or VIS helps alleviate the potential for raking one's knuckles over the forward assist knob, by locating the forward assist assembly further forward on the receiver. As seen from the top, you can see that there's adequate clearance between the forward assist and the right side handle on the Raptor. Another thing to consider is the rear backup iron sight; some sights have large adjustment knobs that you can scrape your knuckles on. All edges are smooth and rounded.
Shown below is the optimal setup (IMHO) for a lefty; a MUR 1S receiver, with only the shell deflector and no forward assist. This leaves the right side of the receiver slick, with nothing to interfere with the operation of the charging handle. You can even use the blade method with this setup. Is the forward assist necessary on a combat weapon? I don't know, but in all the classes I've been to, none of the malfunction clearing techniques involved the use of the forward assist.
Observation and notes - So, back in February, I started using the prototype Raptor. My first impressions of it were that it was very solid feeling, and enabled a straight-back pull without torquing it to the side. My prototype came with different size aluminum handles (instead of steel) which I swapped out to test, and found that the medium latch on the left side and large latch on the right side were probably the best combination. Both provided good traction for the operating hand to grab without being too large or small. The large handle on the right is really necessary when used with standard upper receivers with the forward assist. Monty at Centurion Arms is a fellow lefty shooter, and I felt that he'd be a good person to test the Raptor (since he's active duty NSW), so I introduced Josh to him, who sent him a prototype. About a month later, Monty gave it his thumbs up.
The medium latch on the left and large on the right eventually became the configuration for the production version of the Raptor. The large latch actually doesn't stick out that far - less so that most charging handles with extended latches. The forward angled handles, in combination with the serrations on the front and back provide a secure grip with and without gloves. At 2.4 oz, the Raptor is heavier than a standard charging handle; the steel handles do weigh more. On the gun, the extra 1.2 oz of weight is unnoticeable. That's the weight of two rounds of 5.56mm. When transitioning to a pistol and letting the rifle hang by my side, I did not notice the charging handle digging into my side or snagging on anything. There are other things hanging off the rifle that do that.
The Raptor is made to the same high quality that we've come to expect from AXTS. It's well-designed and robust. As far as ambi charging handles go, it's set the new standard (for me, at least). AXTS is also working on the AR10/SR25 version at the time of this writing and should be available by December 2012.
Update - While I was writing this article, I was in frequent communication with AXTS, providing my feedback on the production version of the Raptor. One idea that came about was to re-visit the aluminum latch/handle idea, and see whether the concern with camming surface wear was valid. So, AXTS made some latches out of aluminum and sent a couple to me to try out. They also performed their own in-house testing on the aluminum latches/handles, to see how they'd hold up to wear on their camming surfaces. To do this, they devised a mechanical test fixture that cycled the handles continuously, and cycled them one than one million cycles with hard anodized 7075 aluminum latches. The latches held up incredibly well, so they decided to make the change and offer the same performance in a slightly lighter package. This is an in-line change, and is transparent to the user. I have been using the aluminum-latched Raptor prototypes and notice no difference in performance between it and the original steel-latched Raptor, except for the slight reduction in weight (1 oz) which is noticeable when holding them in the hand. The aluminum-latched Raptor weighs 1.4 oz and the original weighs 2.4 oz. As with the steel-latched Raptors (and most moving mechanical components in general), it's recommended to put a drop of oil on the aluminum latched version mechanism. The photo below shows the new aluminum-latched Raptors.
Battle Arms Development ETP and EPP
9/13/13 - 'Attention to detail' would describe the Enhanced Takedown Pin (ETP) and Enhanced Pivot Pin; the latest products for the AR15 from BAD (Battle Arms Development, Inc.). Something that we've taken for granted for ages has finally been improved.
Background - The front pivot pin and rear takedown pin are components of the AR15 platform that any AR owner should be familiar with. Press in the rear takedown pin from the left side, pull it out from the right, and the upper pivots away from the lower receiver, allowing the bolt carrier and charging handle to be removed. Pressing in the front pivot pin allows the upper receiver to be separated from the lower receiver. Pretty simple right? It should be. The market is flooded with AR parts from various manufacturers; many using dimensions reverse-engineered as few have access to the TDP. Different manufacturing processes, quality, materials etc; you get the picture. All these factors can contribute to variances in upper to lower receiver fit. Sometimes they can be perfect; other times they can be a loose or tight fit. When the fit is tight, it can be hard to push the pins out; at least one of them. That's when we resort to using bullet tips or other objects to push them out from the left, sometimes slipping and marring the receiver. The pin heads can also be difficult to get a good grip on.
What Battle Arms Development has done is taken a fresh look at these lowly pins that we take for granted, and designed them to make disassembly of the upper and lower receiver as painless as possible.
Description - BAD has designed drop-in replacements for both the front pivot pin and the rear takedown pin. Here's a breakdown of the features:
Enhanced Pivot Pin (EPP) - Installation of the standard front pivot pin can be frustrating if the detent spring extends out of its hole in the lower receiver. Trying to use a needle nose plier to hold the detent while using the curved surface of the pivot pin has probably resulted in many a launched detent that ended up lost somewhere on the floor. I usually clip a couple of turns off the spring myself, or make sure that I have something to catch a detent that has launched, but BAD went a step further and provided a means for holding the detent in place so it can be used to compress the spring, then slid the pivot pin into the receiver. There is an extension of the groove that is milled past the normal detent groove, and an extra detent hole nearer the tip of the pin. A small magnet is placed on the pivot pin on the opposite side, and the detent placed in the new detent hole. Note that the magnet illustrated here is not the one that will be supplied with the production EPP - a different one will come with it. The detent is held by the magnet in the hole, perpendicular to the pivot pin. The detent spring is then compressed, and the pivot pin pushed into the lower receiver where is is then retained by the detent.
Once installed, you can see how the features of the EPP make pushing or pulling it from left to right easier.
Enhanced Take-down Pin (ETP) - The rear takedown pin is installed exactly as the standard takedown pin is installed - by loosening and backing off the receiver extension castle nut and receiver end plate, to relieve tension on the detent spring. The ETP is then installed and the receiver end plate/castle nut are re-installed. I've been able to 'cheat' sometimes, depending on the spring tension, and use a small jeweller's screwdriver to depress the detent enough to rotate and remove the takedown pin, without having to loosen the castle nut.
On an assembled rifle, you can see that the reversed flat cut on the ETP at 1:30 is the most logical placement for it. That's where the index finger naturally grabs at, when grabbing the head of the takedown pin. Also, it doesn't interfere with the thumb when using the safety.
Observation and notes - I have a few upper/lower combinations that are pretty tight, and have often wished that the takedown pin was a bit longer, because I've been able to push it to the right and get it started, then didn't have enough purchase on the head on the right side to pull it out all the way. With the increased length of the ETP and EPP, pushing on the pin tips from left to right extend the pin heads an extra .030", which is enough to get more finger on the pin head to pull it out. The extra length is unnoticeable at any other time, and doesn't interfere with the function or handling of the weapon.
The tapered mushroom shape of the EPP along with the dimples really provide the user with a secure grip, especially with gloves. The knurling and straight cut do the same for the ETP. The magnet assist installation of the pivot pin detent is awesome. Quality is what you'd expect from Battle Arms Development: excellent. The enhancements to such small parts really do make a difference, making the EPP and ETP worthy upgrades for any AR15 lower receiver. Be warned: once you use the EPP and ETP on a lower, chances are you'll want to upgrade all your other lowers.
Raven Concealment TopStop
1/18/14 - The TopStop from Raven Concealment Systems is an upper receiver cover for the AR-15/M4. It is designed to keep the bolt carrier from slipping out of the upper receiver and keep dirt and debris out when the upper and lower receivers are separated.
Background - If you've taken apart your AR15, stored the uppers in a safe, transported separate uppers and lowers in cases or rifle bags; chances are that you've had the bolt carrier slide backwards or fall out along with the charging handle. I store my AR uppers vertically in my safe, and sometimes the ejection port detent isn't enough to keep the bolt carrier forward, so it pops open and the carrier slams to the rear when I'm taking them out or putting them back in. I know - 1st world problems, right? That's more of an annoyance than anything else.
I carry my uppers and lowers separated in my EMDOM TNT-GYM Bag, or SERT Systems C3 case. With the C3 case, extracting the upper usually snags the charging handle, popping it open and the bolt carrier often falls out of the receiver. It happens as well in my GYM bag from time to time. In rifle cases, I've had the carrier come out partially as well. One of the issues with this happening is of course convenience; the carrier has to be re-inserted into the upper receiver. Another is that the oil or lubricant comes off the carrier and gets on the inside of the case. Dirt, sand, or debris inside the case can also enter through the exposed bottom of the upper receiver.
The TopStop from RCS (Raven Concealment Systems) is a upper receiver cover that prevents the bolt carrier from sliding out and debris from entering the lower receiver.
Description - The TopStop is a molded piece of polymer that shaped such that it covers the entire bottom of the upper receiver. The top surface has a raised area that matches the bottom cutout shape of a standard upper receiver. There are two rectangular holes that engage the front and rear lugs of the receiver. The hole in the rear has small raised bumps on each side that engage the rear takedown pin hole in the lug, and keep the TopStop from falling off. The front cutout doesn't have any bumps. The TopStop doesn't slip off the front because distance between the lug cutouts is dimensioned tight enough. The rear extends upwards to cover the back of the receiver, and keep the bolt carrier in the forward position. There's a pull tab on the back to aid in removal of the TopStop from the upper. The small hole in front of the rear lug cutout is for attaching a thin cord lanyard for dummy cording the TopStop inside of a rifle case or discreet bag.
To install the TopStop to an upper, it's simply lined up on the lugs and pressed/snapped on. To remove it, you pull down on the rear tab. The TopStop fit on all the upper/bolt carrier combinations that I have, including my .22 LR upper. Even though there are external differences between some forged or billet uppers, most will have the standard bottom shape in order to be compatible with standard lowers, so the TopStop should fit without any issues. There can be variation between upper receiver dimensions, so you'll find that the TopStop will fit tighter on some, and looser on others. It's dimensioned to work on the broadest range of manufacturers possible. As you can see from the photos below,
Observation and notes - When new, the TopStop can be very snug on the upper due to the bumps on the inside of the rear lug hole. I had to tap the bottom of the TopStop to get it to seat completely on the uppers when brand new. The bumps do wear down slightly upon intial installation; shaving off a bit of plastic, and the TopStop becomes easier to take on and off over time. RCS has not had any reports that they wear down enough such that it becomes loose.
If I had to name a couple of things I'd change on the TopStop, it'd be the location of the rear tab and the lanyard hole. The tab is located in the center of the back 'cap' of the TopStop, which covers the back of the receiver. If the TopStop fits very tightly to the rear lug, it requries a good tug to remove it, by pulling downward on the tab. This bends the rear cap of the TopStop, such that it flexes away from the receiver. While it's solidly built with gussets, and I have no fear of it breaking off, it's probably not the best location for the tab from an engineering/leverage standpoint. I'd have located the tab lower down, right behind the rear lug hole. That way, you wouldn't be flexing the rear and it'd be a more direct transmission of the force to the lug hole. Note that this really only applies if/when the TopStop is very tight.
The small lanyard hole is meant for putting a thin cord through it and retaining it with a knot. 550 cord sheath will also fit through that hole, but a 550 cord sheath knot is too bulky to fit between the TopStop and bottom rear of the bolt carrier - there's not enough clearance for anything other than a thin cord. By moving the small hole 1" forward to clear the carrier, 550 cord sheath can be used. It's easy enough to drill another hole if need be.
I installed TopStops on my uppers and it works as advertised. The carrier is nicely retained, and I don't have to contend with the carrier slipping out or back when storing or transporting my uppers. I've had carriers and charging handles slip out of my uppers during transportation and it's just bothersome to dig around for the parts. While it's just an inconveniece for me, it may have more serious consequences for a professional end user if there's any kind of urgency, and the upper and lower need to be mated pronto. If you store or transport extra uppers separated from the lower receivers, you'll quickly appreciate having TopStops installed on them.
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