DSEi 13: STK Airburst 40mm Under Barrel Grenade Launcher

ST Kinetics has been developing their 40mm ABMS (Air Bursting Munitions System) for a while now. The ABMS Upgrade Kit can be installed in a wide range of 40mm automatic grenade launchers, such as the Mk19 Mod 3, H&K GMG, CIS 40 and STK LWAGL. The company now developed a 40mm airburst system that can be attached to regular rifles.


Airburst system mounted on STK SAR 21 Rifle.

A 40mm grenade is going to have a lot more power than the 25mm grenade found in the XM25 or the 20mm grenades found in some other airburst systems. The limited payload of those small caliber grenades has been their chief criticism. On the other hand they can travel on a much straighter trajectory than the 40mm rounds.

To the soldiers and vets reading this blog, is this a system you would have found useful? Or is the extra weight something you would prefer to go without?


STK under barrel grenade launcher.

Steve Johnson

Founder and Dictator-In-Chief of TFB. A passionate gun owner, a shooting enthusiast and totally tacti-uncool. Favorite first date location: any gun range. Steve can be contacted here.


  • Joe Schmoe

    The idea of putting most of the weight forward and to the side was clearly done by someone who has never held a rifle in the field for an extended period of time. There is a reason why most FCS systems put the system on top of the weapon.

    Take the Israeli version, the MPRS , for example:

    As you can see, the weigh is in the sight which is towards the rear, the controls themselves are on the grenade launcher above the trigger. Since it doubles as the regular weapon sight as well, it doesn’t add significant weight and also has the advantage of doubling as an integrated rangefinder for the shooter.

    This system by STK is clearly a design by committee and not be a field soldier.

    • Micki Mahoney

      Both ways have their pros and cons. However, some points to consider: firstly, if you’ve ever carried a bullpup in the field for extended periods of time, putting some weight forward can be a good thing. Secondly, we don’t actually know how much the STK fire control system weighs yet or how it handles on a rifle. Thirdly, putting the bulky FCS on top can limit the field of view, which isn’t ideal. Remember the pictures of the FAMAS FELIN a few months ago, with the red-dot sight on top, way out of cheek-weld?

      • Tinkerer

        My personal experience with bullpup rifles indicates me that having the center of gravity between the shoulder and the pistol grip allows for the rifle’s weight to be partly supported by the shoulder, which allows me to keep the rifle shouldered for longer time and with with less strain than in a traditional layout rifle, where all the weight is supported by the hands.

        • Micki Mahoney

          When actively firing the weapon, then yes, I’d agree with you. But my own personal experience of moving around on patrol/exercise, (which is what the average user will spend most of their time doing,) the butt was constantly dragged down off the shoulder; the back of the grip ground painfully into the web of the hand; and it took more effort to put the weapon back to the shoulder when it slipped down. After a couple of hours it was annoying; after a day or more, it just became a constant, soul-sapping fight against gravity. (Using the older versions of the SA80 family, mostly the cadet rifle and LSW — no idea if the A2 versions are any better or worse.)

          I do believe that the bullpup layout is the way to go, but only if the design allows for safely firing from the left-shoulder and the centre of gravity is over the pistol grip, rather than behind it. YMMV, of course.

  • Reader

    Last I knew most 20mm grenade rounds reach about 650 yards and the issue with them was the size of the payload not range.

    • Steve (TFB Editor)

      I put that really badly. I mean’t the reach of the round from where it is detonated, not range. They have much greater range than the 40mm system.

  • Lance

    This may be sought after by the Marines they didn’t want the small 25mm Grenade from the M-25 and said they want a 40mm one. This may be what they are looking for.

    • Anonymoose

      Maybe if you can wire this thing to an M32…

      • noob

        well, ST kinetics makes a belt fed, crew served, highly mobile 19.5kg “light weight automatic grenade launcher” that can accept their airburst system.


        it’s not 5kg like the M32, but it could be interesting…

      • Geodkyt

        You can. That’s one of the selling points they list in their literature — it can be added to ANY of the existing 40mm (NATO, not Russian or Chinese) GLs.

    • Joshua

      I don’t understand that. The Marines here lately have been amazing me with their rational. From the 25mm air bursting grenade argument to their decision on upgrading their rifles, their decisions are mind boggling.

      The m25 was actually performing incredibly well in field trials, and the 25mm rounds were doing their job. I see no reason to need a 40mm in this case with how accurate and how close you can place a 25mm and then have it detonate 2-3ft from your target.

      • Patrick Mingle

        Except for those few “I’m going to blow up in your face malfunctions” 😉

        • Joshua

          Ammunition problem.

  • Alex Smith

    I don’t see why we don’t just bring back an oldie but goodie. HE Airburst. It bounces up then explodes like a shooting bouncing Betty.

    • noob

      bounding grenades have a role, but airburst grenades are supposed to be easier to use because they will detonate on target without you having to score a direct hit on a narrow trench. Also airburst grenades can attack people just behind a window frame at a variety of angles and ranges while a bouncing grenade would continue on until it hits an interior wall and is more likely to explode off target.

  • sianmink

    I’d like to see it as a standalone system. Grenadiers don’t want 10lb of crap bolted to their carbine.

    • noob

      I wonder if it would be able to bolt onto a standalone stock that already exists for GLs like the M203?


      • dp

        This seem to me as over all better solution. Of course operator would need a self-defence weapon of sort on top of it; something like Mk23.

        • sianmink

          A stripped M4 would be fine. 6.5lb and you can bum ammo off your squaddies, letting you carry more grenades.

      • DiverEngrSL17K

        Good call. That might just be the next logical step for STK.

      • Tinkerer

        The article’s last pic shows the grenade launcher detached from the rifle, and on the background you can see a stock accesory, apparently designed for a standalone purpose.

    • Geodkyt

      They advertise being able to attach the FCS (necessary to use the airburst grenades) to ANY 40mm GL. So, you can attach it to your standalone GL.

      That and the caliber are why this is significantly superior to ANY of the 25mm systems.

  • DiverEngrSL17K

    While Joe Schmoe’s assessment is sensible and well thought-out, I tend to agree with Micki Mahoney on this one. STK has some very experienced and intelligent designers and engineers on its staff, many of whom have probably served their time in the Singapore Armed Forces. In addition, due to the close cooperation that exists between STK and the Ministry Of Defence by nature of their business structure, it is also possible that STK was able to field pre-production prototypes for trial by selected Army units, with the benefits of direct feedback from the end users.

    No weapon system is ever perfect, i.e., able to offer the soldier the best performance in all aspects of what makes up a weapon system. There are constant trade-offs among weight, bulk, balance, complexity and cost. Designers therefore generally strive to achieve the best possible overall compromise among these factors with a specific set of objectives to tailor the weapon for its intended usage. Sometimes they succeed, and sometimes they fall short and have to re-work or modify the weapon.

    Micki has a very good point about the placement of the FCS low and slightly forward on a bullpup. As far as STK’s set-up goes, it appears to minimize overall bulk, keeps the center of gravity low, and partially counter-balances the rifle since it is located fairly close to the center of balance. It should be kept in mind that the handling characteristics and maneuverability of a hand-held weapon are not just a function of weight reduction alone, but also weight distribution. If you can centralize the mass by keeping it close to the center of balance and maintain a reasonably low center of gravity, the weapon will not only feel lighter than it actually is, but will also be much easier to manipulate quickly.

    Having said all this, the only way to really confirm or deny how well the STK system works is to actually use one for extended periods in the field.

  • Nathaniel

    There are two grammar errors in this article.

  • Joshua

    I hope the thing is free floating
    Given the AUG uses a 8 lugatory bolt like our M4A1’s I feel for them if this thing weighs alot and then gets hung on the barrel.

    The more weight you hang off the barrel the more the barrel extension shifts. The more the extension shifts the more uneven pressure you place on the bolt lugs. The more uneven pressure you place on the bolt lugs the lower the bolt life will be.

    This is one reason why the RIS II had a free float requirement, by FF’ing our M4A1 barrels we saw drastic bolt life and reliability increases in the M4A1. The Aussies also found this when they were testing a FF barrel in their EF88’s.

    • DiverEngrSL17K

      Except for one thing — the FCS on the STK SAR-21 appears to be attached to the handguard. If the 40mm grenade launcher component is also hung on the handguard, it is unlikely that there will be unacceptable shifting or oblique stress issues with the barrel and bolt, even if the handguard and barrel assembly are integrated, i.e., non-free floating, provided that the handguard design provides most of the support at the rear attachment point ( at the front of the receiver ) without depending too much on the forward attachment point to the barrel or barrel surrounds further downstream. Like almost everyone else, I only have the photographs to go on, and visual perspectives are obviously limited, so this is based on available information and photo-documentation.

      It’s a little hard to tell, but the uppermost photograph seems to indicate that the barrel may actually be free-floating anyway, which would address the potential problem you referred to. We really need to get our hands on an SAR-21, complete with the FCS and grenade launcher, to verify this one way or the other. Any other takes on this particular detail, anyone?

  • Joshua

    Also about this part. “The limited payload of those small caliber grenades has been their chief criticism.”

    The only ones who have criticism are the ones who have not actually seen what the rounds can do. The soldiers who got to test the XM25 loved them and the 25mm rounds did their job. The M25 is incredibly accurate and when the soldier does his job the 25mm does its job.

    • Steve (TFB Editor)

      I have only seen product videos of the XM25 and other air burst systems. Its not a question of does it work, I am sure it does, but is it better than alternative systems? I don’t know. Having a good mix of weapons for future unforeseeable conflicts is always a good idea, so I don’t fault the idea of adopting them.

      The 20mm airburst product video really do not inspire confidence. The US Army ruled out 20mm airburst a long time ago.

      With any airburst grenade (at least for now) at best only 50% of the payload will go in the direction of the target. So for a tiny 20mm grenade, half the limited shrapnel and limited chemical energy is heading off in the wrong direction.

      • Big Daddy

        Steve that is the biggest drawback to this system. You are just wasting so much of the round. In a full auto AGL a burst is different. From a single round I cannot see how good it is at KILLING. I’m sure they love it because right now and for the last few years the idea of breaking up an attack was more important than killing the attackers. I just don’t see the one shot system as being an effective and efficient killer. The 40mm is much more so than a 25mm but even the 40mm is limited. In fact the higher trajectory will allow for more of the round on the target. Think about it, it’s coming down like a mortar and exploding within a group not overhead as with a higher velocity round. The old bouncing Betty in a M32 seems like the best for killing the enemy. Not just pissing them off and allowing them to show their old war wounds to family. I want something that kills if I am going to rely on it for my life and have to hump it too. A high trajectory 40mm round that burst down like a shotgun shell seems most effective against infantry, make sure the pellets are stainless steel or something even better so they are assured to penetrate enemy helmets and the cranium itself. That will insure maximum efficiency in the kill zone.

        • Joshua

          So far the M25 has done a steller job of sending more of our enemies to their 72 virgins.

          • Big Daddy

            I hope so but I have not seen any real substantiated reports. Do you have some? I would be very interested. I only know what I did and was taught with a M203 and not much else.

          • hallne

            If it can’t kill skinny malnourished rag wearers. Modern infantry, wearing kevlar + electronic hearing protection would just shrug it off.

          • Big Daddy

            “We disrupted two insurgents on an OP (observation point) and we silenced two machine-gun positions — two PKM positions,” Conley said, describing some of the scenarios he witnessed in theater where the XM25 had been used. “We destroyed four ambush locations, where the survivors fled.”
            As reported by this web site in Feb 2011. My point they did not KILL them. They had no helmets, body armor nothing it just scared them away. Y’all make up your own opinions on it’s effectiveness. In my book it’s overpriced junk.

          • Joshua

            Yeah and it has seen far more combat than those few engagements, There are plenty of kills with the system, also those 2 you mentioned were some of the very first times the rifle saw combat.

            It is working and it is working well(aside from the primer issues). You won’t get gory after math photos on the internet, those things get you in big trouble if they make it to the internet, but the guys who use it love it and have killed numerous with them.

          • Big Daddy

            Show me the money…….I looked and I could not find any story that corroborates what you are saying. If you go on the net and say things without being able to back it up at least with some type of anecdotal first hand account it never happened. The big boys expect first hand knowledge or it’s BS. But I’ll at least take second hand with some type of report. I think it’s a very good philosophy. Either you have something or not, if not it never happened and you are talkin’ BS.

          • Joshua

            You are not going to find the information you seek in open source documents, I hate to say that but they just are not there. I have a good rep over on a few other sites, I am new here though and I was just trying to shine some light on things. Take it or leave it, that’s up to you.

          • Big Daddy

            Neither take or leave it, ignore it is more like it. Nothing personal, I don’t know you, I don’t know who or what you know and I don’t care really. I not trying to be disrespectful I’m trying to say to you if you do not have reliable information that can in some way be substantiated it never happened. That’s the way it is now, due to all the people on the net these days who are full of it. I’m much more flexible with sources but anybody who is a member of a respected web site with professionals would ask the same thing. If you could not give that info you might be banned actually. I’m not a professional so I am more flexible. Just educating you a bit on the reality of the internet these days. No source = it never happened, period. Plus it might save you from looking like a fool, take it or leave it, that’s up to you.

          • Joshua

            Thanks for the info, looks TFB caters more to the keyboard commando crowd.

            I’ll be sure to stick to the websites where people actually know me, thanks for the information.

          • Big Daddy

            Oh please give it up, keyboard commandos…LOL…grow up unless you are already 15 years old. I belong to web sites that only allow professionals and some ex military. That is their by laws, you post some stuff like that without documentation and you’re gone, they will call you on it. If you belonged to any of them you would have known that. I suggest you get rid of the attitude, it’s YOU. Look in the mirror. Those web sites are strict because the information you post could have an effect on someone’s life or death. They are in the field actually doing this, FOR REAL. Posting hearsay and erroneous information can get someone killed. I don’t do it and neither should anybody else, unless you are on that certain AR15 website. Yes stick to web sites were people know you so that you can all talk BS together.

          • Joshua

            I hang out on PF.com, I’ll leave you to figure out what that is, and there I do not need to post classified documents to fulfill the desires of every guy who sits behind a keyboard, because every guy there is the BTDT SF type.

            But hey, I can’t please every guy who demands to get his hands on classified documents that are not allowed on the internet, best I can do is share as much as possible without saying to much.

          • Big Daddy

            I don’t care what that is. You are wrong…..if it is classified you should not be talking about it, period. Do you understand? Can it be made any clearer?

          • Joshua

            Lol so I take it you have deployed with the XM25? Cause the only information you have is a couple of quotes from 2011. Also the documents are classified, just like the documents on the SCAR trials are, I can still make some general comments that does not say much, just like I do about my experiences with the SCAR.

            I did like the bit about how me saying the M25 works will get people killed that was a good one.

          • Big Daddy

            Give it up, the more you talk the more you sound like who you are. I am not going to answer you anymore to just say this place would be better off without you and your types. If you want to litigate become a lawyer, not here and not with me. If you don’t understand what I am saying so be it. I really don’t care. Did I say I did any work with the XM25, no you just want to argue. You are one of those who just like to argue on the internet with people. If anybody is a keyboard commando it’s you. I can tell you were never in the military they would have figuratively beat this childish BS out of you in basic training. Or maybe given you a blanket party and done it for real, that’s what they do with your type.

          • Joshua

            Wow, clearly you are the voice of TFB.

            Look man I have no idea why you seem so mad, or what your problem is. You quoted a statement from 2011 and I stated something about how the 25mm airburst does its job and somehow you became irritable and agitated.

          • Big Daddy

            I suggest you re-read the thread and what was posted. Like I said you like to start things with people, it’s obvious. If you are older than 30 your behavior is an indictment on our society. I called you on your BS and you don’t have anything to back it up but more BS and childish nonsense……I won’t post anything more on this so you can have the last say as I know you will, you must, you cannot help yourself, you must push that button…LOL.

      • Joshua

        Thanks for the reply Steve. The limited field trials were incredibly successful until the double feed that brought to light ammo issues.

        The 25mm is a good round and performs well against targets. It is also incredibly more accurate than a M203 or M320 allowing for shots through windows that I have never seen made by a person using a 40mm launcher.

        From all the data gathered in the field the 25mm is an excellent choice for a low recoil(well less than a similar 40mm), long range, accurate air bursting round.