GemTech GSBC Suppressor Bolt Carrier under Evaluation

Interesting goings on going on, and one of them is constant tinkering with the Mk 18 carbine in the SOF world. One of the things people are doing is running them suppressed sometimes, and not suppressed other times. The word is that this bolt carrier helps make that change in a regular, direct impingement AR like the Mk 18.


The Mk 18 (or the CQBR upper for the M4A1, which produces the same functional weapon) is widely issued within SOCOM and somewhat beyond it. For example, Marines who need such a carbine have them, but Uncle Sam’s Military Club runs them with some different accessories than the SOPMOD gear commonly used in the other branches’ SOF.

Running suppressed is more and more widespread (in conventional forces as well as in SOF). But there are several downsides to a suppressed DI AR. Taken together, these add up to one of the key impetuses to the development of the piston HK 416. But experience has shown arms developers that it’s possible to make a DI AR run well, while suppressed; what has been a challenge is to make the same AR run equally well with the QD suppressor on or off.

To recap the problems:

  1. More pressure than designed into the gas system, yields…
  2. More blowback out of the ejection port, plus…
  3. Much higher carrier velocities, producing
  4. Higher perceived recoil
  5. Higher cyclic rate on AUTO
  6. Reduced reliability, and
  7. Reduced durability.

Gemtech’s solution is so simple that the instructions for using it are pretty much contained in these two box cover illustrations:


The valve flange is on the left side of the bolt carrier. To change it, then, you must remove the BC from the firearm. You can then turn the valve flange to (S) for Suppressed or (U) for, you guessed it, Unsuppressed.

When you’ve made such a change, or, for that matter, at anytime the GemTech Suppressed Bolt Carrier is installed, an indicator visible through the ejection port shows whether you’re configured to run Suppressed (S) or Unsuppressed (U). gemtech03That’s pretty much it. The setting indicator arrow points aft to S, or forward away from S, and makes the whole system fairly Ranger-proof.

The GemTech bolt carrier is adjusted with a flathead screwdriver, but other tools will work in a pinch. The valve can get a little gummy.

gemtech02The GSBC comes with the carrier key screwed and staked in place, but otherwise it is a bare carrier. It is conventionally notched for use with a forward assist. It lists for $249 and can be bought direct from Gemtech or from Gemtech dealers.


Here’s a close-up of the flange where the valve can be adjusted. gemtech06

Gemtech’s claims of reduced carrier velocity and reduced cyclic rate are supported by an analysis by Philip Dater, available on the Gemtech website (.pdf). The reduction was significant on several different weapons, but much larger (25%) on an M4A1 than on a Mk 18 (16%). Still, that’s not trivial.

24 thoughts on “GemTech GSBC Suppressor Bolt Carrier under Evaluation

  1. Tierlieb

    I’ll admit to be a piston snob, so this question may be off, but: Why not simply use an adjustable gas block?
    Adjusting a gas block covered by a rail system is not always comfortable (depending on the design of the rail and position of the gas block) , but taking out the bolt carrier is neither.

    1. Hognose Post author

      Because you can’t retrofit an adjustable gas block to the gun you already have in under one minute! The Gemtech bolt is a drop-in solution.

      1. Tierlieb

        Ah, of course, I had not thought about that.

        I had, however, not expected the need to retrofit existing rifles because I would not deploy a weapon system that does not work reliably in both conditions before I solved that issue.

        But that’s just me. My tribe probably includes the inventors of the Johnson 1917 or the MAS49, guns that were mostly bug-free and already obsolete on delivery. ;-)

        Footnote: The Oberland Arms OA15 M10 is a nice integrated upper (for 300 BLK or Whisper RUAG in CIP country) with the gas block switch integrated into the rail : – allegedly what the Dutch SF buy.


      Cause its prtty hard to beat the factory milspec’ed FSB/gas block for durability and toughness. And adjustable gas blocks are subject to all kind of crap

      1. Tierlieb

        > Cause its prtty hard to beat the factory milspec’ed FSB/gas block for durability and toughness.
        From an engineering standpoint, that would mean that it would be better to weaken the sturdy gas block than the more fragile bolt carrier, wouldn’t it?

        > And adjustable gas blocks are subject to all kind of crap
        Adjustable gas blocks in general work well on a lot of rifles (since the FAL at least) and most piston AR-15s manage quite nicely.
        Low-profile, adjustable gas blocks on DI AR-15s are the problem, because you need to build them so tiny to fit under a standard rail. So you get tiny screws and fiddly adjustments. That I will agree with.
        Something like the JP Enterprises JPGS-6 does not have that problem and is SF-proof (maybe not Ranger- nor Marine-proof).

        So it depends on how big the retrofit is supposed to be. For a minimal one, as Hognose pointed out, the BC is a good choice. For a serious one, I would personally chose the gas block and match the rail.

        1. Hognose Post author

          The FAL gas block was later than some others, like FNs own M1949 and the Soviet SVT. Not sure about the French 1949. The FN version is highly accessible and hand-adjustable (the Tok needs the combo wrench).

  2. W. Fleetwood

    “Fairly Ranger-proof”? I’ll bet it ain’t. Hold my beer. hand me that screwdriver, and watch this!

    Sua Sponte.

  3. DSM

    I’m planning on picking one of these up but in these pre-election bonanza times I’ve been stocking up on the consumables.

  4. SPEMack

    “Fairly Ranger Proof?”

    Do they mean tab or scroll Ranger?

    Monthy Python references aside, this makes me want to go do operator stuff with my friends.

  5. Matt

    At first glance, I’d expect that the gas regulator would cause pressure buildup at the carrier key and cause it to act like a little piston itself. This may not be enough to accelerate the bolt but I’d be interested to see if carrier key/screw life is reduced. Also, I would think that all the built up gas in the tube would still vent inside the receiver once the bolt began its rearward motion and pulled the key off the tube. No?

    I have M16A4 and XM16E1 builds that I put together because they interested me. I shoot them but they aren’t my go to AR15’s. Every AR15 I’ve built that isn’t a reproduction of a certain model has an adjustable gas block. For $30 or so more than a basic gas block, it is perhaps the biggest upgrade for the money compared to a basic build.

    For entertainment purposes, here’s a pic of the best example of a high-function, low-cost build I’ve done. I ordered the parts and helped a coworker put this together. It came to $1,420 out the door though I have a type 1 FFL so it may cost others more depending on their dealers/suppliers.

    1. Chris W.

      Care to share a parts list on that $1,420 rifle? I love to see what other guys are using for parts. That info really brings the geek out in me when I can delve into the details. Maybe Hognose could do a regular post about specialty builds and the parts involved? I dunno, don’t want to add another burden, I know his time is precious and he’s spread a bit thin right now as it is. Maybe someone could point me to a site that does this regularly? I’m not looking for your average PSA rifle, nor the “accurized” Noveske (nothing wrong with those), but the tricked-out (where it counts) rifles without a lot of time spent on the Magpul accessories but more on the internals and how they have affected both the rifle build/cost as well as accuracy/functionality. There’s a ton of stuff on the market that is so gimmicky that it makes me sick. If it’s not “Elite Tactical” then it’s crap right? So we’ve been told (sold?).

      1. Matt

        Sure! I’ll group them by brand to make it a little easier.

        Magpul: Enhanced trigger guard
        BAD lever
        M-LOK AFG
        K2+ grip

        Anderson Manufacturing: upper and lower receiver
        Buffer assembly

        Fortis: Rev 2 12″ M-Lok
        Muzzle Brake

        Strike Industries: Ultimate dust cover enhanced
        Forward assist

        Faxon: 16″ Gunner

        Superlative arms: adjustable gas block

        Aero precision: 30mm Ultralight scope mount

        Vortex: Strike eagle

        AXTS: Raptor charging handle

        ALG: QMS trigger

        Griffin Armament: QD lock plate

        and Lancer: L5-AWM magazine

        Some thoughts about the build. This was the first semi automatic I’d shot with a scope on it and it was extremely easy to aim at the target. Along with the muzzle brake, it had virtually no recoil to disrupt your point of aim and made quick, accurate, and repeated hits on target a breeze. I had worried that the muzzle brake would make shooting it unpleasant but was only fractionally louder than my M16A4 build which obviously has an extra four inches of barrel and no brake. I’d do the brake again in a heartbeat.

        Some items, like the lock plate and gas block were used simply because they were cheaper than the competition and not because they offered any particularly special performance.

        For a lefty, I’d add the troy ambi mag catch and the AXTS Talon but that’s about it.

        The ALG QMS is inexpensive but I found it to be smooth and crisp enough to make accurate shots with and yet heavy enough to be suitable for something besides range work.

        If by some cosmic coincidence you happen to live in Utah, let me know. I enjoy helping people put together guns.


    Very clever. But I am very skeptical

    I am curious what one would do in the field if the can some how HAD to come off and the gun used unsuppressed but you don’t have time to get your tool out and take your gun apart.

    seems to me the time it takes to make the change on the BC could also be used to pop in a new buffer and spring. and that requires no tools. And I doubt neither of those operations would be a common thing to do in the field.

    this reminds me in a lot of ways, of that absurd idea of the LMT rifle that allows you to swap barrels from rifle to carbine to sub-carbine etc. with just a tool. As if some one would do that on a mission like LMT suggested.

    I am not a believer of piston ARs being the wonder cure either. Over rated. The vaunted 416 and SCAR have a lot of their own issues people tend not to talk about.

    1. Matt

      The ideal solution would probably be an adjustable gas block with a few gross adjustments. It would need to protrude from under the hand-guard and be adjustable with a bullet tip. The problem you then encounter is what I have heard with other easily adjusted gas blocks in military service. To wit, boots cranking it up to the highest setting for faster cyclic rate/reliability. That however, is a training issue that may or may not be acceptable to attempt to fix depending on your viewpoint.

      1. Hognose Post author

        Or you get the Bolivian situation, where, with nobody in the unit ever live firing anyway, literally nobody below officer rank knew what that little jobbie below the FAL sight did. And when we introduced them to the joys of live firing, some thought their guns were jamming when they had the gas port set for grenade launching (gas cut-off)!

  7. Will

    I have two of these, and have been testing them for some time.

    In response to Matt’s comment about restricting flow, they do not in fact do this. The little valve is just a port out the side of the carrier, so when it’s set to suppressed mode, it is exhausting additional gas out the ejection port, rather than restricting flow like an adjustable gas block. The downside to this is that left handed shooters can end up with an entire face full of gas (not to be confused with the normal gas-to-face effect of adding a suppressor, which comes out the charging handle area). When you flip the switch to unsuppressed, the vent is closed, and it acts like a normal carrier.

    If you have the carrier set to suppressed mode, and remove the suppressor, the gun may eject, but won’t pick up another round. I’ve tried it on a LMT MRP 10.5″, BCM 11.5″, BCM 16″ (midlength gas system), and Noveske 18″ (intermediate length gas system) with both A5 and carbine length receiver extensions and associated buffers.

    Also of note, the staking on the gas key is terrible, and the screws are marked YFS, which is a Taiwanese company with unknown specifications, rather than grade 8 fasteners like Colt, LMT, etc. use. The Gemtech carrier does appear to be almost identical to the Bootleg carrier (sand cuts on contact surfaces, aesthetic milling), which AFAIK is made by PWS. The Bootleg carrier has additional settings, not just suppressed and unsuppressed, but some of the forum feedback I’ve seen suggests that the fully suppressed setting doesn’t vent quite as much gas as the Gemtech. The Bootleg carrier is also adjustable from the ejection port; you don’t have to remove it from the rifle.

    Personally, I find LMT’s enhanced carrier to be a better option for rifles that get shot both suppressed and unsuppressed, as it requires no adjustment, and has a modified cam pin track which delays unlocking. This greatly reduces blow back from the barrel. Rate of fire is also reduced in full-auto, although the last time I had access to an M16 lower earlier this year I only had regular Colt and BCM carriers to compare against.

    Looserounds – I’m a big fan of your site, too! Regarding the LMT barrels in the MRP and MWS platform, you’re right, it is kind of silly to think someone would swap barrels in the field, not to mention the change in zero, or lugging around those extra barrels which weigh quite a bit. They are very neat platforms otherwise, since it is easy to change the barrel and caliber. The use of a the LMT enhanced carrier or Gemtech/Bootleg carriers is a nice option on the MRP, since the barrels have angled gas ports and a proprietary non-adjustable gas block.

    1. DSM

      I’m glad you brought up the LMT enhanced carrier in comparison to the Gemtech unit. Are there any issues with using them in a Mk18 upper? I seem to remember some time back LMT actually had an admonition against using them in a 10.5″ upper but don’t see the same warning now.

      1. Will

        Several years ago LMT had a warning that the enhanced carrier/bolt were designed for 14.5″ barrels or longer, but this is no longer the case. The latest revisions don’t seem to have any issues with SBRs, and there’s plenty of users on that confirm this. You can also email or call Gene at LMT if you have additional questions. I have over 5000 rounds through a BCM 11.5″ with an enhanced carrier, almost all suppressed, and am steadily running the count up on my LMT MRP 10.5. I’ve been swapping back and forth between the Gemtech and LMT carriers for the last few months. No issues whatsoever with either one, as long as the Gemtech is set appropriately. I did replace the gas key and screws on the Gemtechs with Colt parts, and stake them with a MOACKS. It’s ok to call me paranoid. :)

        If you have a real MK18 upper with the crane spec gas port, you shouldn’t really need a different carrier. The gas port size was chosen for 5.56 pressure ammo, and should run well even suppressed by bumping up to an H3 buffer.

        On the other hand, if you have an off-the-shelf Daniel Defense MK18 upper, the gas port is huge, and the LMT enhanced carrier or Gemtech SBC should make a big difference. People insist on shooting weak ammo like PMC bronze or Wolf/Tula steel case, and the manufacturers get tired of hearing how a gun won’t cycle reliable so they drill out the gas ports much larger.

    2. Hognose Post author

      Great points, Will, thanks for chiming in. As it was explained to me, the Gemtech did without any capacity to change it through the ejection port because they wanted to make it impossible to have an uncommanded change of setting. (Perhaps that’s something prototypes may have done in testing?). There could also be dueling patents at play here.

      1. Will

        Makes sense. The Gemtech setting selector is only retained by the inside of the upper receiver, so unless they added a detent or lock of some sort, making it adjustable from the ejection port might result in some issues. I’d be curious to see if the Bootleg added a detent to keep the selector in place.

        Also, interestingly, the Gemtech is rated for full-auto, but the Bootleg is not.

    3. Chris W.

      Awesome write up. Thanks for the info, I’ll be researching these for days or weeks I’m sure!

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