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Posted: 7/25/2026 7:35:53 PM EDT
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I keep going back and forth on pulling the trigger on 37mm. I would like to hear from those of you who own or have experience with 37mm as a platform. To be more specific. Im considering either the LMT m203 37mm or the iwa ml37. For me this is just a toy. I have zero practical use for such a device. Hows the quality/availability of 37mm ammo? Do the 26.5mm adapters work reasonably well? Is it as fun as you expected? Whats something you wish you knew before you made the jump? Etc. Thank you for your time. |
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I had a Cobray (sp?) for years. It was a lot of fun. Sent up a lot of flares, experimented with chalk, had a 12ga adapter for it -for marine flares and it was useless. A longer adapter would have been very useful, which leads me to my next comment: I also am looking at the LMT 37mm for one of my 20 inch A2 clones; but a Master Key would be way more useful and practical. Just sayin .02 YMMV and all that ... |
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I have both 37 and 40mm It was easy to find 40mm chalk. I did find some 37mm baton rounds which were fun. The whole 37mm non-rifled complete is not a firearm, to non-bareled receiver is a firearm to 40mm is a DD is funny to me, but hey that's the letter of the law. |
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I have an ML37 that I'd like to DD and posdibly convert to 40mm. Most of what I have shot through it are various bird bangers that I've designed on my own. It is pretty satisfying having something like that work as intended. I still should DD it so I don't have to be so careful about what I load in it. The side opening launchers like the ML37 or the M320 (if it happens) clone can take longer rounds than a forward opening M203. If all you shoot is 40mm chaulk, that won't matter. If you like to experiment with oddball rounds, it might make a difference. |
The government is just a corporation with a monopoly on violence.
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Originally Posted By Master_Blaster: I have wanted a 40mm UGL to mount on my ARX100, but its proprietary rail makes it a nonstarter.  Only thing I can think of is acquire the Beretta PIC rail (which is aluminum and 5" long but at least hasn't got the interrupted segments) from Beretta itself or from Brignoli Armi and then add one or more top/bottom PIC rail adapters (Picatinny to Picatinny Rail Clamp) below that to mount an M203A2 rail mount GL. Several outfits make PIC-to-PIC adapter clamps of various lengths. I dunno if your ARX100's polymer receiver interface would soon show harm being done to it but I doubt it would be horrible if you were only shooting Cheetos or light loads like rubber ball rounds out of a 12g adapter. You might want to stay away from 3" magnum 000 shot. |
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Originally Posted By PvtCowboy: No improvements since we last talked about this before three years ago then? Only thing I can think of is acquire the Beretta PIC rail (which is aluminum and 5" long but at least hasn't got the interrupted segments) from Beretta itself or from Brignoli Armi and then add one or more top/bottom PIC rail adapters (Picatinny to Picatinny Rail Clamp) below that to mount an M203A2 rail mount GL. Several outfits make PIC-to-PIC adapter clamps of various lengths. I dunno if your ARX100's polymer receiver interface would soon show harm being done to it but I doubt it would be horrible if you were only shooting Cheetos or light loads like rubber ball rounds out of a 12g adapter. You might want to stay away from 3" magnum 000 shot. I inquired with a couple of M203 mfgers & no joy. That add doesn't specify the material, & it looks like the poly lower rail that I've got, which wouldn't work imply due to the insufficient material strength (hence the geometry of the rail on the ARX's lower HG). |
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Originally Posted By Master_Blaster: That add doesn't specify the material, & it looks like the poly lower rail that I've got For $25, not a lot to lose to give it a whirl. |
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Originally Posted By PvtCowboy: When I did a little sleuthing for you the other night I did read that buyers confirmed that Beretta factory rail is indeed aluminum. Okay, for that price I don't expect its going to be from a forged 7075 ingot, but who knows? Maybe they are and Beretta Defense is just blowing them out. For $25, not a lot to lose to give it a whirl. Really? I'll try to find out what grade it is. This makes me wonder what it might cost do design a piece on Fusion & have it milled in a suitable material like 7K aluminum. Hmmm... ETA just received a reply that it's the same polymer rail.
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| Running out of ideas, but here goes: Get the polymer rail, confirm it at least fits your ARX100 satisfactorily, then send it away to SendCutSend to have a duplicate made from 6061 T6 Aluminum. |
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Originally Posted By PvtCowboy: Running out of ideas, but here goes: Get the polymer rail, confirm it at least fits your ARX100 satisfactorily, then send it away to SendCutSend to have a duplicate made from 6061 T6 Aluminum. That's an avenue. You think 6K series has sufficient strength? |
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Originally Posted By Master_Blaster: That's an avenue. You think 6K series has sufficient strength? I would think shot weight is the biggest factor in this. The typical M781 Cheeto practice round is only a few ounces lighter than an M406 HE round and doesn't have anywhere near the kind of fierce recoil like 3" Magnum 12g buckshot rounds do out of my very short barreled (like 3") 12g adapter. I am usually launching from my XM177 length carbine which is pretty light even with the GL attached so recoil is a bit of a punch. |
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I’m your huckleberry…. 6061 T6 is the industry standard on handguards and rail systems used on AR platform rifles. It should handle any 37/40mm round with no problems. For testing it, make sure your mounts are a match to your rail system. No play or slop in the connect points. Quality is worth the extra few bucks, especially for mounting hardware. The cheaper rails out there are made from softer aluminum, and will fail. When you first get your rail mounted launcher set up, start by firing one round, and check the rails closely for any shift or damage. Then two rounds, same thing. Check for tightness in the mounts and hardware. Then five rounds, remove handguards and an inspect everything again. Give the launcher a few hard yanks (haha) and check for looseness. Ten more rounds, same thing. Check your launcher before and after firing the same way whenever you fire it after, just to ensure everything is tight. GROG |
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@master_blaster I just played with SendCutSend app's custom builder using the picture of the Beretta ARX100 bottom rail and it made a really accurate pic rail model with locator pin in the proper position. They quoted a reasonable price for it made from billet 6065 T6 with economy of scale pricing for multiple copies that may set you up in a little cottage industry for the small number of people who want an M203A2 rail mount for their own ARX100. Now, the AI generated model measurements below are just roughed out and inexact in the Pic rail segmentations and rear locator hole pin size but tuning it to fit is relatively simple with a micrometer and the genuine Beretta polymer part in hand. You might find it best to sketch out a one piece solution of modified lengthier rail size to mount an M203 on your ARX100, but you have a ballpark pricing idea now. Get one satisfactory prototype completed then tell SendCutSend to build you twenty of them for resale after you send them off for anodizing/blackening/laser engraving. ARX100 M203 Mount by MasterBlasterCo    |
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Originally Posted By PvtCowboy: @master_blaster I just played with SendCutSend app's custom builder using the picture of the Beretta ARX100 bottom rail and it made a really accurate pic rail model with locator pin in the proper position. They quoted a reasonable price for it made from billet 6065 T6 with economy of scale pricing for multiple copies that may set you up in a little cottage industry for the small number of people who want an M203A2 rail mount for their own ARX100. Now, the AI generated model measurements below are just roughed out and inexact in the Pic rail segmentations and rear locator hole pin size but tuning it to fit is relatively simple with a micrometer and the genuine Beretta polymer part in hand. You might find it best to sketch out a one piece solution of modified lengthier rail size to mount an M203 on your ARX100, but you have a ballpark pricing idea now. Get one satisfactory prototype completed then tell SendCutSend to build you twenty of them for resale after you send them off for anodizing/blackening/laser engraving. ARX100 M203 Mount by MasterBlasterCo https://i.postimg.cc/yYrx3d4J/IMG-3377.jpg https://i.postimg.cc/QxnCKtG9/IMG-3378.png Man, you are the man! Looking into it, 7075 is nearly 2x stronger than 6065 & since the M203 receiver is 7075, I was initially thinking that I needed to use the same for the rail until I realized that HG pic rails are 6065, so really no problem there. I've actually thought about having the rail cut with just 3 slots to match the LMT interface, with the idea being to improve strength, with the goal of mitigating the chance of any of the slots breaking under stress. There is, however, 1 problem: the slot that the retaining pin goes through is actually elliptical, as seen here:  While the accessory pic rail uses a round clevis pin (of sorts), the GLX160 uses what appears to be a proprietary elliptical pin that seems to be intended to eliminate play, as seen in this image (2nd object from top, below the standalone mount & above the GLX160 receiver):  I'm not sure how to accomplish the same with a standard round pin. If this is necessary, & the part can't be sourced (Beretta won't play), then the only feasible option would be to adjust the hole interface in the rail so that a round pin aligns with the back half of the elliptical receiver hole interface when inserted & maybe put some fill material in the remaining space. Assuming this is a necessity. Thoughts? |
| Go with 40mm and register it as a DD. Lots of best ammo out there. Bengalbullets (however you spell it) has point detonation rounds that are really epic, close as you can get to an actually exploding round. Way more fun than chalk rounds IMO. |
"I prefer dangerous freedom over peaceful slavery". - Thomas Jefferson
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Originally Posted By Spartikis: Go with 40mm and register it as a DD. Lots of best ammo out there. Bengalbullets (however you spell it) has point detonation rounds that are really epic, close as you can get to an actually exploding round. Way more fun than chalk rounds IMO. All for it. Just need to figure out this detail, but I think it'll work. |
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Originally Posted By Master_Blaster: I've actually thought about having the rail cut with just 3 slots to match the LMT interface, with the idea being to improve strength, with the goal of mitigating the chance of any of the slots breaking under stress. There is, however, 1 problem: the slot that the retaining pin goes through is actually elliptical While the accessory pic rail uses a round clevis pin (of sorts), the GLX160 uses what appears to be a proprietary elliptical pin that seems to be intended to eliminate play, as seen in this image (2nd object from top, below the standalone mount & above the GLX160 receiver): https://www.berettadefense.com/wp-content/uploads/2022/03/glx160-standalone_bdt_zoom002-1-850x278.png I'm not sure how to accomplish the same with a standard round pin. Thoughts? Lastly I'd keep an eye on the doggone Airsoft custom market to see how Asian gamers are mounting GLs to Beretta ARX160s. These solutions they dream up aren't going to be military grade materials but they are sometimes geometrically correct(ish) to the genuine article. If you could pick up something that works you could CAD model it off to SendCutSend. There are several of us who bought perfect Japanese knockoffs of Knights M203A1/2 RDS tangent sights that are bloody identical to the real thing for a fraction of the price. They do amazing things over there and they do make ARX160 Airsoft rifles. Here's some pics of how the Irish did it for the M203PI and how the Australians mounted theirs to vintage AUGA1s before they switched to Lithgow-Thales AUG clones to give you inspiration:   |
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Originally Posted By PvtCowboy: My thought is that you're not trying to mount a GLX160 but an M203A2, so you would focus on a newly fabricated Pic rail to match the rear locator hole with a round pin and shim and then bolster it in front using one of the barrel cross pins near the gas block. How you'd do this is with a modified CAD design that drops the pic rail interface another half inch or so which allows room for the rail segment to extend out another three inches longer. Then you'd use a horse collar up towards the muzzle end of the rail that shrouds the barrel like the various contraptions used in how the M203 was retrofitted to the Steyr AUG for the Australian and Irish armies. From your ARX100 I see some likely looking cross pins at the gas block that may satisfy it and anchor the rail in place. Lastly I'd keep an eye on the doggone Airsoft custom market to see how Asian gamers are mounting GLs to Beretta ARX160s. These solutions they dream up aren't going to be military grade materials but they are sometimes geometrically correct(ish) to the genuine article. If you could pick up something that works you could CAD model it off to SendCutSend. There are several of us who bought perfect Japanese knockoffs of Knights M203A1/2 RDS tangent sights that are bloody identical to the real thing for a fraction of the price. They do amazing things over there and they do make ARX160 Airsoft rifles. Here's some pics of how the Irish did it for the M203PI and how the Australians mounted theirs to vintage AUGA1s before they switched to Lithgow-Thales AUG clones to give you inspiration: https://photo.weaponsystems.net/image/s-carousel/n-iw_gl_m203pi_p01.jpg/--/img/ws/iw_gl_m203pi_p01.jpg https://pbs.twimg.com/media/GJHC_dHXQAArZBh?format=jpg&name=medium Good ideas. Been ruminating on this. The accessory rail uses a simple round, grooved clevis pin, retained by a spring clip, through the oblong hole at the back of the receiver. The extra space not filled by the pin would create longitudinal slop/play in the rail, but for a slot in the receiver located below & aft of the oblong hole the a tongue extension on the accessory rail locks into to keep it in place, with the pin preventing the rail from sliding forward & off. This arrangement works for a molded rail, but it likely wouldn't work for this application, especially considering the GLX doesn't utilize it. The oblong slot is the problem. The GLX160 uses a complimentary oblong cross section clevis pin that completely removes play in the mounting assembly, but I can't find that style of pin commercially, so it's likely proprietary, or else my search-fu isn't up to the task; Beretta certainly going to throw me a bone. Fabbing a rail using a normal round cross section pin, the insertion point should locate it against the back end of the oblong hole, but the remaining space allows the rail to slide forward a bit, which is no-go if the GL were to be fired, allowing the rail+GL assembly to travel a few mm back & slam against the receiver. No bueno. There needs to be a method to prevent it from sliding forward. Barring a breakthrough finding an oblong shaped pin, I've come up with 2 ideas to prevent the rail from sliding forward, with the aft pin positioned against the aft side of the oblong hole: 1) Fill in the extra space not occupied by an inserted, std profile clevis pin with a (removable, if needed) hard rubber sealant to eliminate forward play, or 2) Incorporate a 2nd hole in the rail that aligns with 1 of the pic rail slots molded forward of the proprietary rail & run a clevis pin through that slot to prevent the rail from sliding. Both pins lock the rail in place, with the aft pin bearing the recoil load of the GL. The single caveat is that the spec pin being oblong makes me question if load bearing strength is a factor here, & if a normal round profile pin will be sufficient m. |
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Attached File If the features indicated by the red arrows are the oblong pin and holes that you're talking about, then that should be an easy part to sketch up and have machined for you. I can't tell if there are subtle features in that pin that would complicate reproducing one, but it doesn't look like it. I am curious how it is retained while it is installed. This is just an educated guess on my part, but I'm pretty sure that the (very approximate) blue lines are mating surfaces that would bear a lot of the recoil. The stock I mounted my ML37 to wasn't designed with those in mind, so the fit isn't perfect. The back of the receiver rests on the surface above the pistol grip, but there is some forward slop. I installed a Magpul rail mounted sling loop just forward of the launcher, and that keeps it tight. Not a great picture of that but it is the best I could find. Attached File |
The government is just a corporation with a monopoly on violence.
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Originally Posted By Gingerbreadman: https://www.ar15.com/media/mediaFiles/145837/glx160-standalone_bdt_zoom002-1-850x278_-3811426.JPG If the features indicated by the red arrows are the oblong pin and holes that you're talking about, then that should be an easy part to sketch up and have machined for you. I can't tell if there are subtle features in that pin that would complicate reproducing one, but it doesn't look like it. I am curious how it is retained while it is installed. Here's the ARX100 proprietary rail that the proposed adapter would mate to:  There's no rear bearing surface even with the GLX160, & particularly when its mounted on the ARX:  Mounting this is going to be a different ballgame:  That pin is likely some form of wire or ball detent clevis pin. Assuming such a custom part could be fabricated, what might it cost? Specifically, a flat head clevis with a ball detent would be needed. Also, I'm wondering, what wall thickness would the adapter need to provide sufficient strength (under load) where the pin interfaces the upper receiver & the adapter? WTA, I'm starting to notice how the angled region in front of the mag well that houses the barrel retention mechanism will force the M203 to be mounted farther forward because the trigger housing sits directly behind the barrel, whereas the GLX trigger is behind & below the barrel, allowing it to clear the slope. Besides mounting it farther forward, the only other workable remedy that would allow an M203 style GL to be mounted fully at the back end of the adapter rail would be for the design to have it hanging significantly lower, which would look & handle rather awkward.  I saw a GLX160 fs on gunbroker once, way back in the 2000's. I should've jumped on it. 
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I forgot that you were trying to adapt a rail mounted M-203 to it, my mistake. If you are going to design an adapter rail that will need to be machined, the cost of adding that pin to the order is going to be a rounding error. I really don't think that the pin is critical in terms of absorbing recoil, but that's just an educated guess. If they flipped it 90 degrees so that the flat sides were perpendicular to the bore, then I'd be more inclined to think it was important. If it is just there to keep the launcher from sliding forward, then you could design the adapter with a round hole located to keep a round pin against the forward edge of that slot. |
The government is just a corporation with a monopoly on violence.
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Originally Posted By Master_Blaster: Barring a breakthrough finding an oblong shaped pin, I've come up with 2 ideas to prevent the rail from sliding forward, with the aft pin positioned against the aft side of the oblong hole: 1) Fill in the extra space not occupied by an inserted, std profile clevis pin with a (removable, if needed) hard rubber sealant to eliminate forward play, or 2) Incorporate a 2nd hole in the rail that aligns with 1 of the pic rail slots molded forward of the proprietary rail & run a clevis pin through that slot to prevent the rail from sliding. Both pins lock the rail in place, with the aft pin bearing the recoil load of the GL. The more challenging design would be cantilevering a sufficiently long rail to mount the M203A2 to the barrel for support. Just so you know this solution has always been an armorer's clever idea using an array of pipes sleeved over the barrel or sheet metal brackets and whatever's laying around that looks likely. |
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Originally Posted By Gingerbreadman: Attached File This is just an educated guess on my part, but I'm pretty sure that the (very approximate) blue lines are mating surfaces that would bear a lot of the recoil. Concur. Resuscitating this discussion, what 12a SG rounds can the 203 receiver handle using an adapter? I read that slugs & 00 are out, so what's the upper limit? Is it down to birdshot & assorted Aguila light loads? Chalk rounds seemed like a no-brainer, but sheeeot they're spendy. 
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Years ago, a good friend made me a set of adapters I still have and use. Every gauge, from 410 to 10 gauge. I’ve fired them all on video. The 10ga I used turkey loaded magnums. No issues. Standard mounted M203. Not rail mount. Also fired quite a few experimental rounds and helped Scot Pace of Pace Defense develop his Cannon rounds and 4x410 adapters. Some of those had pretty substantial recoil. No issues. GROG |
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Originally Posted By benglesbullets: For the cost of a LMT 37mm or a ML37 I would get a 40mm instead. The LMT M203 37mm is non standard and it's very hard to find ammo for. Not sure where you’re looking, but there are all kinds of 37mm loads available from a variety of retailers. And, in some cases, they’re significantly cheaper than their 40mm counterparts. |
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Originally Posted By grog18b: Years ago, a good friend made me a set of adapters I still have and use. Every gauge, from 410 to 10 gauge. I’ve fired them all on video. The 10ga I used turkey loaded magnums. No issues. Standard mounted M203. Not rail mount. Also fired quite a few experimental rounds and helped Scot Pace of Pace Defense develop his Cannon rounds and 4x410 adapters. Some of those had pretty substantial recoil. No issues. GROG Just purchased a 12ga adapter from Scot. He recommended limiting rail mount launchers to 3" shells max. My concern stems from the reports of the firing pin, latch, or even the receiver itself failing from the higher pressures. Would 2-3/4 slugs be safe? |
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