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Lima 6 and AR Long Stroke Piston Conversion (Page 1 of 3)
Posted: 11/28/2025 11:45:42 AM EDT
[Last Edit: tommyidone][Edited]
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Hey All, Im deep into prototyping stages of a Long stroke Gas piston system for AR15's. It's something I have been entertaining for a long time now and with the advent of the LIMA6 its finally coming to a head. When L6 was announced I jumped on that train asap as I have wanted a belt fed for a LONG time but just could not stomach the cost of an upper in their current form, not to mention all of the quirks and compromises. Frt's being commonplace now has changed the practical capabilities of AR's and their uses especially with a beltfed upper. With long stroke gas piston systems you are able to keep the heat and gas away from the carrier and feed mechanism. Arguably the same can be accomplished with a short stroke system but that would include extra parts and a new carrier or bolt on anvils/strike face for the op rod which don't seem to last very long. Im pretty close to getting prototypes made in steel and am finalizing some small parts and details. Because this has been designed for a standard AR15, the space is limited between the bore and gas tube centerlines in the upper receiver and that limits what I want to do with design. Currently the design works for all gas block journal sizes up to .875. If I were to say make blocks only for .625-.875 I could somewhat optimize the design around those journals. To make it for .936 puts it on the tight side for the adjustment mechanism that I'm utilizing. Its going to be a 4 position system by the way. Im super excited to get this working for my personal projects. Should I abandon the .936 size or keep the jack of all trades? Ill throw some pics up of the latest revision when I get back from thanksgiving out of town. Thanks to all! |
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That block looks rad, and the piston sounds promising! So far as barrels go, I don't really see the utility of going over .875. How many barrels are out there over .875? And it would be rad if someone does an adjustable with a large enough control surface to do with out tools, or a bullet. Like an OG SAW |
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.750 and .875 are the bread and butter for sure. .936 is possible but requires some pretty silly compromises IMHO that I don’t think are worth it. The op rod to bore centerline on the AR is just not really conducive to much bigger than .875 or .750. The is going to be field serviceable and adjustable without tools. The block is essentially done I have made some tweaks and changes with serviceability and adjustment in mind and this setup is going to be killer. I am very pleased with the adjustment mechanism. It feels rock solid and has extremely positive engagement. All of the critical dimensions have been worked out and I’ve figured practical tolerances. Working on drawings now but that shouldn’t take too long. The hard part is figuring out how to get an op rod with a piston attached through the upper receiver. I have a plan to tackle this problem with minimal or no changes to the upper. It just might require another small step for takedown but it will be simple. There are other methods I’m working on right now and have some jigs built and am waiting on some tooling to do some mockups with m4 uppers. |
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Originally Posted By DH2: Do you think there is any advantage to shoot for compatibility with the riflespeed gas block/adjustment system? Not that I can think of other than I wouldn’t have to make it. I think in general this setup will be more robust. The adjustment plug is dead simple to replace without tools. If over thousands and thousands of rounds you find some erosion on the plug, you simply change it out. Op rod will be easily replaceable as well. The last hurdle I’m working on is ar15 compatibility. I want to use this on my current AR’s. L6 is side charging but I want to try and get this to work with standard charging handles as well. Once I get my L6 upper in hand I can confirm that everything is going to work and get the op rod and carrier anchor made. It’s just going to take a bit more thought. The last thing I want to do is go off half cocked and order parts that end up not working hundred percent how I want them to luckily I’m on the first batch for L6 so hopefully that wait is not too much longer. |
| The less compatibility you have, the less acceptance you will have. There's a tremendous # of gas blocks & charging handles out there - if you don't have the ability to use them, there is less incentive to try your system. |
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Duly Noted. Thanks for the input. I really appreciate it. small changes to be made. Its actually going to be easier than I thought. shopping for fab shops. Im having trouble getting pictures to load after I upload them to the site. Attached File |
| If your system was clamp on & used standard gas blocks, you could use an even shorter piston, & could be fed by a short gas tube from the gas block - but then you'd need the ability to get under the handguards. |
I sell firearms produced by the finest child labor in the world, be it Filipino, Muslim, Mormon, Arizonan, or Texan.
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Originally Posted By backbencher: If your system was clamp on & used standard gas blocks, you could use an even shorter piston, & could be fed by a short gas tube from the gas block - but then you'd need the ability to get under the handguards. Originally Posted By backbencher: If your system was clamp on & used standard gas blocks, you could use an even shorter piston, & could be fed by a short gas tube from the gas block - but then you'd need the ability to get under the handguards. That’s an interesting idea, I hadn’t thought of that. I think this will be more sound and no gas tube to deal with. Originally Posted By what_manual: Looks good. Adjustment looks robust and decent amount of settings. Should add hole for easier pinning. Not sure what that block of material is for (main body) on the left though. It could be a clamp on if one wanted to do that but I’ve always been a fan of pinned blocks. And if someone didn’t want to pin it for whatever reason, two set screws are there. I could upsize those to 1/4-28 or something if people are worried about it being secure?. I have a habit of overdoing things lol. |
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Attached File In that pic that’s a .980 heavy barrel with an .875 gas journal and carbine length gas system. I toyed with some things back and forth including not having a gas tube or rather a very long one. I’ve actually made quite a few changes to the gas block and the carrier anchor. I’ve removed 3 parts and two different features and was able to streamline things good bit. The good news is this looks like it could fit under a BAR style handguard. I chose this mainly because it’s big and would be easy and there’s a few different manufacturers that make them. With some changes to the barrel nut which is specifically for a bar style handguard I think other handguards could be possible. I’m still committed to getting this running on standard ARs as well as the Lima6. I’ll reach out to the L6 guys about some dimensions to verify some geometry with them but in any case hopefully I’ll have my L6 upper in soon. Which handguards are people using nowadays. It’s really annoying that there are no real standardized barrel nuts for free float handguards. |
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That think looks great! If it could be made to work on a Lima 6, I would prefer this over the rifle speed, just based on the way it adjusts So far as handguards go, there are enough out there with shovel noses, or block cutouts. So long as the piston fits under the handguard. |
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It looks really good. My only concern would be the spring for the retainer button giving out under extreme heat. I see you've made that area larger in the latest rendering... (Maybe going with a larger spring?) which is a good idea. Another idea you might consider is a flat external spring, like HK switched to in their latest adjustable gas blocks for the 416. Not sure about patent protection, but it has already been copied. The downside is you may not have room. Might have to use 416 handguards in order to use something that tall (and keep it under the handguard).  https://nefariousarms.com/products/na5-hk-416a5-style-adjustable-gas-block |
| I’ve completely eliminated the button altogether but retained the indexing system. The spring was actually a concern of mine prior to this revision. I feel like it would have been fine but this is slimmer and uses a much larger indexing surface with a much larger 17-7 ph spring now. It is one of the higher temp rated springs I could find in the format I wanted. It is also secondary in its application for locking and doesn’t rely on it for positive locking of the gas position. There are other examples of systems using much smaller springs with small detents that function well. Thanks for the feedback. I do plan on doing some pretty intensive testing on this. |
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I have two different approaches to attach the op rod to the carrier depending on how the internal geometry of the l6 is laid out in the gas key/ “charging handle” area. I don’t use threads in the carrier key. I wanted to eliminate any stress risers and it gets pretty thin in there. I want this to be bomb proof. In one case it is an interference fit and a solid pin. It’s going to be a custom carrier anchor in both cases. I’ll be able to determine which of the two designs I will use for certain once I get my l6 upper or once they get back to me with some dimensions. Or it might be a different op rod/carrier anchor for the l6 Here is a mock up of it. It fits under an aero bar style handguard. Attached File |
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Originally Posted By backbencher: Is the op-rod just going to be screwed onto a standard bolt carrier, or are you going to have a unitized carrier/piston rod? To clarify the custom anchor/key and op rod do not rely on the screws to transfer the energy rearward. The anchor has an integral lug (or two) that indexes on the carrier to take shear forces. Looks slightly similar to how pws sets up their op rod and carrier key but I really don’t know how they attach or anything or how the op rod is fitted into the key or that to the carrier. |
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Originally Posted By tommyidone: To clarify the custom anchor/key and op rod do not rely on the screws to transfer the energy rearward. The anchor has an integral lug (or two) that indexes on the carrier to take shear forces. Looks slightly similar to how pws sets up their op rod and carrier key but I really don’t know how they attach or anything or how the op rod is fitted into the key or that to the carrier. Originally Posted By tommyidone: Originally Posted By backbencher: Is the op-rod just going to be screwed onto a standard bolt carrier, or are you going to have a unitized carrier/piston rod? To clarify the custom anchor/key and op rod do not rely on the screws to transfer the energy rearward. The anchor has an integral lug (or two) that indexes on the carrier to take shear forces. Looks slightly similar to how pws sets up their op rod and carrier key but I really don’t know how they attach or anything or how the op rod is fitted into the key or that to the carrier. Makes sense. Since you're doing long stroke instead of short stroke, there will be much less stress on the interface than a short stroke boss. |
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| It occurs to me that you might be able to adapt the bolt carrier from gas to short stroke piston with appropriate measures. I know that a bolt on piston strike face will eventually shear, however if you designed a strike face that was also pinned to the bolt carrier through the gas channel hole, you ought to reduce the shear forces on those screws. Using a heavy duty split pin instead of a dowel should also make the positioning of the strike face pin hole vs the gas channel a little less tricky to machine. |
| I had thought about that and even went so far as to draw one up and play with it a bit. Adam’s arms at one point made a bolt on strike face but discontinued it and digging deeper it seems they were prone to failure. There’s a reason why everyone that is still around uses an integral strike face. If the op rod gets even a few thou too much run up before it hits the carrier it will start deforming it. While it may possibly be simpler to do, Ultimately I don’t think it will be as durable. The short stroke setup could eliminate maybe one part/assembly step but I’m looking for bomb proof and really feel like this is the best way to go about it difficult as it may be. |
| There’s on last detail I’m contemplating and that is how to prevent someone from accidentally removing the gas plug. The removal notch is very apparent when you hit it and the plug will want to try to come out. I don’t think you could do it and accidentally not know. But I’m wondering if y’all think adding a small detent on it to give you a sort of hey you’re about to hit the removal notch feel. Would it be worth it or not? |
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The failure point was the screws shearing as those were the only points of contact between the key and carrier. Using a heavy duty split pin, or a dowel pin if you can get your positions accurate enough, that uses the gas hole in the carrier will alleviate the stresses from the key screws. While you are completely correct that an integral strike face is more durable, question is if a bolted and pinned key is durable enough for the application. If it is, then it becomes possible to adapt existing guns to use a short stroke piston without requiring manufacture of new carriers. Which in turn will keep costs down. |
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Originally Posted By DH2: The failure point was the screws shearing as those were the only points of contact between the key and carrier. Using a heavy duty split pin, or a dowel pin if you can get your positions accurate enough, that uses the gas hole in the carrier will alleviate the stresses from the key screws. While you are completely correct that an integral strike face is more durable, question is if a bolted and pinned key is durable enough for the application. If it is, then it becomes possible to adapt existing guns to use a short stroke piston without requiring manufacture of new carriers. Which in turn will keep costs down. PWS does it and I have never heard of one of their carriers and op rods coming apart. Since I last posted I have been running some numbers and looking around. It's hard to find good data on what they do to retain their op Rod to the carrier but I found some pics of a cutaway gun they made. I feel extremely confident that my solution will be just as durable as their setup. They utilize a recess milled in and a lug in the gas hole. Mine utilizes two lugs. Also being that the carrier/oprod and piston are essentially one piece you won't see any of those impact forces for an op rod slamming into a strike face. The peak force should be very much within the shear capacity of 2x 3/16 lugs made of 17-4 PH steel. The strength of the material in sheer is anywhere from 110-140 ksi. the cross sectional area of the lugs plus the material strength tells me that it should withstand something like 6k lbs of force before it lets loose. It seems like massive overkill to me. I don't believe my math is wrong but I think the margin for safety is quite robust even under dynamic loads. There are some interesting things I have read about shot peening surfaces that might be interesting to employ for additional strength. |
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How about a L6 Height Pic riser to run an iron up front and a shorter hand guard? It's been back and forth with small things and looking for shops. Took quite a break from the project the last few weeks. Every time I change something I end up going through all the drawings again and double checking things. Long process but I think it's paid off. Im quite happy with how its turning out. Attached File Attached File |
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Originally Posted By WestOfMcAllister: I've been following this for a while now. Love to see the progress. Have you thought about a Front Sight Post like on the Fightlite MCR's? Basically, instead of a pic rail on the gas block, just have a machined FSP that can be dove-tailed on? I have thought about that. It was one of the rabbit holes I went down early on and even designed a fixed and a forward folding one. I ended up deciding to go this route because of the numerous options for fixed or folding front sights out there. I wanted to have more options vs less and this was the easiest way I could see to do it. Any standard AR flat top front sight will work on this. Or you could omit the pic rail and riser entirely and run without a front sight on the block. The worlds your oyster. |
| The machinist had sone ideas for some small changes to make these easier and cheaper to manufacture. I am meeting with him next week to go over the changes. If I’m going have these made I’m gonna have to figure out how many would be interested in purchasing. If I can have them done at a larger scale that only helps with cost. If I can garner enough interest this would be worthwhile and pretty rad if I don’t say so myself. |
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Changed a few small things for DFM, I had a few people ask about a pic rail on the 9 o'clock so I added that in. Any feedback would be welcome. The version with the pic rail will not fit under a hand guard obviously as it's at the correct height for an ar15 flattop upper. For my use with a shorter hand guard leaving the gas system exposed vs buried this works just fine. Here is the latest modeled up fitting an L6 receiver stub. Thanks to Harvey @Type7SOT for getting me that model. What do you all think? is there anything missing that you maybe would like or maybe something you don't like that you would like changed? This is the last chance for me to change things before I start making real parts. Attached File |
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| I know the red right hand guy uses a proprietary barrel nut for his hand guard. I actually tried contacting the gentleman that makes them to see if I could get one to ensure cross compatibility, but I never heard back from him. I’ll reach out again and see what I can do. It’s a bit early yet but once I get a hand guard from him, it’s very possible that I can make it work with minimal changes. |
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Originally Posted By tommyidone: I know the red right hand guy uses a proprietary barrel nut for his hand guard. I actually tried contacting the gentleman that makes them to see if I could get one to ensure cross compatibility, but I never heard back from him. I’ll reach out again and see what I can do. It’s a bit early yet but once I get a hand guard from him, it’s very possible that I can make it work with minimal changes. Definitely make it work with the RRH handguard. Or at least a compatible version |
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Originally Posted By SGT-Fish: Definitely make it work with the RRH handguard. Or at least a compatible version plans are in the works to hopefully shoot for compatibility with the RRH hand guards. Waiting for quotes from Machine shops to come back for the Piston kits currently. I should have a good idea of pricing after that. |
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I received some initial quotes and prototypes should be ordered by the end of next week or early the week after. Things finally seem to be moving. Im shooting for 5-10 prototype kits. I have two L6 Uppers to test on. The remainders will most likely be AR15 uppers unless I can get my hands on some more L6 uppers. I have spent an enormous amount of time trying to get these prototypes dialed in so I don't have to make any big changes to make it to production. Im satisfied and I think all that work will pay off. Ive been able to incorporate all my goals into it. I designed this to work with Full profile .980 barrels. Up to .875 gas blocks, 4 Position tool-less adjustment and removal (patent pending) and mid and carbine length kits. Now I get to wait... Again. |
Lima 6 and AR Long Stroke Piston Conversion (Page 1 of 3)
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