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Posted: 5/31/2025 2:52:15 AM EDT
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I picked this XTR-12 up from Anthony at Evlutionz,LLC He has taken it to what it is today, an 8 inch SBS He tried to work with UTAS in this creation but didn't have any luck convincing the engineers of needed changes. I have not taken it out and sent some slugs down range as of this writing. Evl said it only likes hot ammo or to be run with a barrel extension to get more dwell time. It has a somewhat odd gas piston system that I am going to change. It has a small diameter gas piston. I believe they are slow to accelerate and push the bolt and carrier to a functioning speed. Thus requiring more gas at higher pressures. Like too small of a garden hose. I want to make a new gas block with a larger piston and move all of it closer to the chamber. The upper/lower fit is not good. Lots of side play. Verticle is good. Having this much side to side play, causes lost energy. How much I don't know. May be part of the problem. It also has a free floating op rod. I plan to fix this issue. It will be a lot of work designing, machining, configuring, and fixing whatever else I run into. I bought a new XTR-12 and a parts gun to work with this one to assist in getting it to function. I will break it down and post some pictures to show the internals and what I feel are the issues I need to address. But for this initial post, just the gun as it is today.   |
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Opening thoughts. I decided to open up the XTR 12 and show the guts of it. To see if I could figure out why it is not compatible with a short barrel, in this case, the 8" barrel. I'll show the parts and comment on what I see and what I think could be the issue, and what I might change. All of this is preliminary and mostly just my thoughts. Anthony at Evlutionz,LLC did a lot of work on this gun, it is an early production gun. Some, if not all of this design is to work in the AR platform. Thats difficult to adhere to with a shotgun as the room is limited. UTAS chose to put the gas system on top of the barrel, under the handguard instead of around the barrel where there is space to achieve that. So no gas piston running on the barrel. No gas rings on the barrel. A separate gas block and piston is what they came up with. Here are the main componnents. The upper, handguard, barrel, gas block, gas piston, op rod, and the bolt carrier and bolt.  Another view, more detail.  Parts separated-better look at the gas block. Notice the 14 gas holes.  Gas block, long op rod, piston, gas cap, and location on the barrel.  A shot of the ends.  The end of the gas block where the piston stops. Look closely and you will see bulging of the piston stop point. The slightly lighter part on the shelf. Piston was driven hard. This is evident. Piston is only .469 in diameter.  The bolt carrier has be lightened quite a bit, I would say extensively. Nicely done as well.  , Few more pics for reference.   Bottom of the gas block.  The muzzle end of the gas block  Gas block/piston reference  Quick summery A lot of gas ports says a lot of gas was needed. That being said, there is high pressure and high volumn. Not sure at this point where I'm leaning. The piston is too small of diameter for the load it has to move. It also has a roughly 1/4 stem on the back of it. That doesn't help. This is a long traveling piston, about 2 inches. Most shotguns are less than 3/4 of an inch of travel. Saiga 12's less than 1/2 inch of travel. The volumn of the gas area is too large. It could be less, even with this design. This is the smallest single piston I've seen in a shotgun and I believe the load of the carrier is too great to accelerate to achieve the travel needed to cycle. Anthony also stated that a 5 round mag works with hotter ammo, but a 10 round doesn't without a barrel extension added. 10 round mag has greater spring pressure, more drag for the carrier to move the shell out of the mag, thus it won't cycle. Lightened carrier? Won't cycle now, how about a heavier buffer spring to strip the shell out of the mag? Worse! At this point, a larger diameter piston is needed. Shorter traveling piston with more horsepower (surface area) Roughly 2 inches of barrel in front of the gas ports. Not enough dwell time. Re-design the gas block and move it closer to the chamber. And that op rod is 6.25 inches long and has some unneeded weight. Thats got to go. Different design here as well. I thought the op rod might fall out of the gun at full bolt retraction since the original capped end is gone. It didn't. Remember this is an early version and I don't have any idea as to where a revised version ended up. Going to look into that. If I recall, the production guns liked hotter ammo even with a long barrel! I welcome any comments readers might have. |
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Wow, I hardly ever think about this stuff anymore. I used to eat, sleep and breath short shotguns. It was definitely my passion. God had other ideas. I trust His judgement. Brother, I hope you move on this. I'd love to see it run some weak loads proper. I ran out of time. The failure of the XTR12 started with them simply modifying an existing high-power rifle action design to accommodate a 12ga round with no consideration of reciprocating part/s weight and the fact that 12 does not have the pressure needed to move that high-power rifle weigh carrier. They doubled down on failure by allowing the handguard to dictate the gas piston size and not letting the gas piston size dictate the handguard design. It looked nice, not reliable though. I ground and ground on that carrier for a couple days, taking almost everything I could from it. It was the single most effective modification. It would not even stovepipe before that. I filmed a lot of the testing and review of the footage each time after adding ports showed further and futher diminishing returns. There just wasn't enough piston surface area to use the additional gas. This is what i wanted to do with it... 1- Bolt Carrier group: Compare the carrier to a standard AR10 carrier and figure out what is closer, the Armalite design or the LR308. After identifying which carrier would be best, buy a titanium carrier. The weight of everything about that bolt carrier group was designed for a high pressure cartridge, not a low pressure round like 12ga. The question of will the XTR bolt fit and function in a regular carrier or a modded one needs to be answered as well. 2- Piston and Op Rod: I wanted to make a Titanium one piece piston/op rod like in an AK or a two piece that can be interference fit and roll pinned together to eliminate the op rod falling out if the gas system was relocated. Diameter would be the same as a Saiga-12 piston Save a little weight and potentially allow relocation of the gas block. 3- Gas Block: I wanted to make a gas block for it that would use a Saiga-12 gas regulator, it's just a good simple design that works. |
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Originally Posted By evlblkwpnz: Wow, I hardly ever think about this stuff anymore. I used to eat, sleep and breath short shotguns. It was definitely my passion. God had other ideas. I trust His judgement. Brother, I hope you move on this. I'd love to see it run some weak loads proper. I ran out of time. The failure of the XTR12 started with them simply modifying an existing high-power rifle action design to accommodate a 12ga round with no consideration of reciprocating part/s weight and the fact that 12 does not have the pressure needed to move that high-power rifle weigh carrier. They doubled down on failure by allowing the handguard to dictate the gas piston size and not letting the gas piston size dictate the handguard design. It looked nice, not reliable though. I ground and ground on that carrier for a couple days, taking almost everything I could from it. It was the single most effective modification. It would not even stovepipe before that. I filmed a lot of the testing and review of the footage each time after adding ports showed further and futher diminishing returns. There just wasn't enough piston surface area to use the additional gas. This is what i wanted to do with it... 1- Bolt Carrier group: Compare the carrier to a standard AR10 carrier and figure out what is closer, the Armalite design or the LR308. After identifying which carrier would be best, buy a titanium carrier. The weight of everything about that bolt carrier group was designed for a high pressure cartridge, not a low pressure round like 12ga. The question of will the XTR bolt fit and function in a regular carrier or a modded one needs to be answered as well. 2- Piston and Op Rod: I wanted to make a Titanium one piece piston/op rod like in an AK or a two piece that can be interference fit and roll pinned together to eliminate the op rod falling out if the gas system was relocated. Diameter would be the same as a Saiga-12 piston Save a little weight and potentially allow relocation of the gas block. 3- Gas Block: I wanted to make a gas block for it that would use a Saiga-12 gas regulator, it's just a good simple design that works. Hi Anthony Wow, so glad you jumped in here. I was hoping you might see this post. I have an extra Saiga 12 gas block from a messed up parts gun I bought. Gas block, piston, carrier and bolt, 23" barrel, cover, FCG, and a carved up receiver with all the rivited parts in excellent shape. The Saiga gas block barrel diameter is .910, the XTR is .940. going to turn down the XTR barrel for the Saiga block. Going to move it closer to the chamber, as close as I can all things considered. Over an inch closer to the chamber. Hotter gasses, higher pressure, all good things. Going to start with 4 gas ports like the Saiga. Turn a temporary clamp on sleeve to cover the existing gas ports. Don't want to weld them up at this point. They will not be covered by the gas block located closer to chamber. Going to use the Saiga piston and add a contained op rod so it can't go anywhere. It will guide the op rod. It will have two diameters. Second diameter is its stop. The op rod housing will interface with the saiga block and get as close to the upper receiver as possible with good alignment with the op rod hole in the upper receiver. Going to stay with the piston travel used on the Saiga. It's .458 inch of travel. It's short but the diameter is effective in moving that Saiga 12 bolt and carrier. Should move this bolt and carrier as well. By using the Saiga block, I can get the piston .015 closer to the barrel centerline. That will help with centering the op rod. Another option is to make a new gas block similar to the Saiga, but optimised for the XTR with set screws like the XTR to secure it. Easy to disassemble for changes. The center of the piston (I think) will be a bit higher than the center of the op rod opening in the receiver. Op rod alignment to the receiver will be critical in my design. Not to touch the upper receiver at all. If the alignment gets close enough, I will make a one piece piston-op rod as you mentioned. Have not measured this yet. Op rod probably 440C steel. It will never deform. The right titainium would also work. My thoughts are that a lighter piston/op rod (Titainium) has less mass, may not carry the load as well as a heavier part (Steel) I've seen this with not being able to strip rounds out of a larger capacity magazine. This has not been proven by me but it has my attention. Mass, acceleration, and velocity calculations is not my expertise. I'd fumble that ball pretty quick. I will weigh the Saiga 12 bolt-carrier and the XTR bolt-carrier to find what I'm dealing with. They may be close in weight but I think the XTR will be lighter after all of your lightening work. That will be in my favor with the 8 inch barrel. I will be gaining a fair amount of dwell time by moving the gas block closer to the chamber. I have another un touched bolt/carrier at standard weight I may throw in to compare the operation of them. No other changes. This sums up my thoughts at this time. Absolutely open to your thoughts, ideas after seeing where I'm headed. I won't be able to use the XTR hand guard as the gas block will be a bit higher than the hand guard by a fair amount. The opening on top of the hand guard will have to go all the way to the receiver. Will have to find another way to mount it if I try to use the factory piece. Keep checking in on this one as I will post pictures as I move forward. Check out my other projects on this SBS page if you get a chance. |
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Time for another update. Putting on hold use of the Saiga 12 gas block. The XTR gas system is not adequate to to operate the XTR in a short barrel configuration. The gas piston is just too small. Doesn't have the horsepower to drive the bolt/carrier as needed. Even with the 14 gas ports on this barrel, there is no more acceleration available. The pedal is already on the floor. Pushing harder won't go any faster. The piston needs to be of larger diameter and that is where I've gone at this point. I caniballized a gas block form an American manufacturered 12 gauge AR platform. Quite different firearm but still uses a DPMS lower (.308) The Intrepid RAS 12. The gas block on that gun sits in front of the handguard with a piston/oprod of 12 5/8 inches long. Thats crazy long. I've decided to do some machine work and adapt it to the XTR. I'm scraping the original XTR gas block. After a lot of measuring, there are three critical dimensions needed to make this happen. First is the barrel diameter where the gas block must sit. 1. On the XTR, the barrel diameter is .940. Thats measured where the gas block currently sits. The Intrepid RAS 12 gas block sits on a .930 barrel diameter. Thats .010 smaller. The XTR barrel needs to be turned down to acomodate the smaller diameter gas block of the factory RAS 12. Thats what I did. The barrel is now .930 diameter and the RAS gas block slides on nicely. I also machined that diameter about .200 longer, closer to the chamber. This allows to fit the longer gas block and me to adjust the gas blocks position to a starting location. The gas port in the RAS block is not in the same location as the XTR block. Thus I needed some room to locate the gas block over the gas ports already in the barrel. 14 of them are preseent. The new gas block is centered over the first 5 ports in the barrel, closest to the chamber, and the rest are blocked by the gas block itself. This is where I'm starting out. Will those 5 ports supply the gas needed to drive the new piston/rod configuration? Also, I don't know if the location on this barrel is the original location of the gas ports or if they have already been moved closer to the chamber. 2. Important numbers: The XTR gas piston has a diameter of .469 and it travels a total of 2.115 inches. Thats a lot of travel, a lot more than needed and the gun still won't function properly. That is also increasing the functional time of the gun. The RAS gas piston is .618 diameter and it travels just .915 of an inch. Travel is 57% less than the XTR travel. And an increase of 32% in piston diameter, 32% increase in power to move the bolt/carrier and at a faster rate. Just to compare, the Saiga 12 gas piston is .825 diameter and travel is at .448. Short travel but big diameter piston with a lot of power to move the Saiga 12 bolt/carrier 3. Bore centerlines of the two gas blocks. After checking them both, what I found was a more than pleasant surprise. The gas bore centerline relitive to the barrel centerline (the distance between) on the XTR was .802. These bores run parallel to each other. The gas bore centerline on the RAS is .846, a good deal different, further apart. But still parallel to the barrel bore. The gas centerline is .044 higher. The piston on the XTR is separate from the op rod for a reason. It does not land in the center of the piston. Its offset by .044 of an inch. Can't be connected to the piston. The piston/op rod on the RAS is a one piece design because the op rod is centered in the piston so it can spin freely as needed. This also give great stability to the performance of this critical part. Always straight, always aligned with the corresponding componnents. What does that mean? The XTR block is low enough to fit inside of the handguard, There is substantial relief in the top of the handguard and the gas block barely fits under it. The RAS block, on the other hand, was designed to sit in front of the handguard. The top of the gas block is the same height as the top of the pic rail on the handguard. So this RAS block will not fit under the handguard. That will have to be dealt with once I get this gun running on 1200 FPS ammo. This all happens because of the need in piston diameter to function the gun is greater than what will fit under the handguard. UTAS definately missed the boat on this design. UTAS chose not to listen to Anthony about needed changes to make this gun perform as an SBS. Note. The previous mentioned dimension of .846 is the same dimension from barrel C/L to receiver op-rod centerline. That made the use of this RAS 12 gas block on the XTR a perfect match for this conversion. Both the XTR receiver and the RAS receiver are nearly identical at the front of the receiver. The rest of the upper receivers differ somewhat. So this is what it looks like as of today. Asembled for the pictures.  ,  , The gas ports on the barrel, the port on the gas block, the relationship.  , The gas block will use just these 5 ports to begin with so it has been moved to line these up with its port. Note: The RAS 12 barrel has 4 ports. Two are bigger than the other two. But even the smaller two ports on the RAS are larger than the ports on this gun.  , The RAS 12 barrel gas ports and the RAS 12 gas block and op rod/piston combo at 15 inches long, piston/op rod combo is 12 5/8 inches long. A new op rod/piston is being made, the op rod will be roughly 5 5/8 inches long. If this configuration won't run, everything gets moved closer to the chamber and a shorter op rod. The machined gas diameter length increases, moving closer to the chamber, the op rod gets shorter, new gas port will need to be drilled at the new location. A nice increase in dwell time will result of moving closer to the chamber. Hotter gasses, higher pressure, cleaner burning, less maintenance. This gas block will not be adjustable. Its cap configuration won't allow modification. I'm working for use of 1200 FPS slugs, 1 oz, and 1200 FPS shot shells, 1 1/8 oz loads. I will tune for reliability with those loads. I will use a press fit sleeve over the existing ports to block at this time, but still removable. There is only 2.5 inches of barrel beyond the existing gas ports. If I have to move everything closer to the chamber that number will grow, a good thing, more dwell time.  , The RAS 12 upper and its gas block location. 18 inch barrel.   Today, I cut the piston/op rod from the Intrepid RAS 12 to the desired length for use in this firearm. Didn't wait for the new one to be made. Fit is perfect and the XTR 12 is ready for testing. Being at this point happened pretty quick, faster than I anticipated. Going to try and get out tomorrow and see where I'm at. July 6th. So I'm working somewhat in the dark, some unknowns, and I've never fired this weapon. Seems like a lot to surpass. I've got some input from Anthony that gave me his direction, even before I made the purchase of the XTR. I'm fortunate to have an RAS 12 to steal a gas block from thats pretty close to what I wanted. If all of this fails, I have a Saiga 12 gas block with an auto plug. Its gas block barrel diameter is .910, even smaller. Just a little bit more machining on the barrel and would need to create a housing to retain and guide the op rod which would also need to be created. I welcome any ideas for a handguard since the gas block won't fit under it. It will be different. I'm also looking for 2 more UTAS XTR 12 magazines. tough to find as I've not found any. 
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I may have confused you on the story of that gun. I bought that one to see what it would take to make it run as an 8" SBS. I knew they didn't run well at full length and I used to love challenges, lol. The first time I felt the weight of that bolt carrier I knew at least one reason of why it didn't run. The piston diameter was another one of those moments. I went through the gun and figured out the main issues that were killing the potential of the platform. When I contacted UTAS they said they did not plan on a Gen 2 and had abandoned the platform altogether. It could have been one hot shotgun for AR guys and could have really went somewhere. Don't worry about the op rod not contacting the piston dead center. I had one seriously misaligned Saiga-12 6.5" build that the trunnion was way out of alignment when the hole for the barrel was machined, didn't affect ejection at all, but the barrel was angled upward so much that it had feed/extraction issues that took a lot of effort to resolve. 5 ports @ 0.080" (angled, not perpendicular like that RAS barrel) is a good starting point for that barrel length, 0.080" is the sweet spot for gas ports in a shotgun, in my experience. They generally do not clog easily and they are still small enough to make larger shot cup/wad debris not go into the gas block. Bear in mind, the bore diameter of the XTR is about .724' and Saiga-12 is about .718". There will be slightly less resistance and pressure/time on the larger bore barrel. I'm pretty excited about this, that's a rare occurrence for me these days  |
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Originally Posted By evlblkwpnz: I may have confused you on the story of that gun. I bought that one to see what it would take to make it run as an 8" SBS. I knew they didn't run well at full length and I used to love challenges, lol. The first time I felt the weight of that bolt carrier I knew at least one reason of why it didn't run. The piston diameter was another one of those moments. I went through the gun and figured out the main issues that were killing the potential of the platform. When I contacted UTAS they said they did not plan on a Gen 2 and had abandoned the platform altogether. It could have been one hot shotgun for AR guys and could have really went somewhere. Don't worry about the op rod not contacting the piston dead center. I had one seriously misaligned Saiga-12 6.5" build that the trunnion was way out of alignment when the hole for the barrel was machined, didn't affect ejection at all, but the barrel was angled upward so much that it had feed/extraction issues that took a lot of effort to resolve. 5 ports @ 0.080" (angled, not perpendicular like that RAS barrel) is a good starting point for that barrel length, 0.080" is the sweet spot for gas ports in a shotgun, in my experience. They generally do not clog easily and they are still small enough to make larger shot cup/wad debris not go into the gas block. Bear in mind, the bore diameter of the XTR is about .724' and Saiga-12 is about .718". There will be slightly less resistance and pressure/time on the larger bore barrel. I'm pretty excited about this, that's a rare occurrence for me these days This looks pretty good assembled. I think it actually might run 1200 FPS 1 1/8 shot load. Got a lot of Remington Gun Club. Got some Noble Speed at 1300 FPS, 1 1/8 ounce load. You sent me some barrel pieces and they are at the machine shop getting threaded adapters made for each. 2" to 7" lengths with the Saiga thread, 22X.75mm. Probably don't need all of them. I have a lot of the Saiga and Vepr 12's I'll be working on. They might come in handy. Nice to see the centerlines of the gas block and the receiver matched. Looks to be the same on the other DPMS uppers I have. I went with a one piece piston/op rod set-up as you had mentioned. Didn't want the rod being in a two piece being able to angle itself so much, loss of energy. If I end up with the two piece, the rod will be retained in a housing right in front of the gas block and having two diameters for retention. Next step: If this configuration doesn't perform, I will move the gas block another 3/4 to an inch closer to the chamber, drill 5 new ports like I circled on the barrel. Add about 30% more dwell time moving it 3/4" I do have a question on the lower you might have an answer for. There is .020 side to side slop, upper to lower. The upper fits pretty good on my other lowers so this lower has the extra play. Was this done by you, that it was needed, or it just came that way because it is painted/finished? I have a fix to tighten it up but thought you might have set it up this way for a reason? Anyway, I'm going to give it a run as it is with no barrel extension. |
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Time for an update. I went to the indoor range Jan 8th. Spent an hour there with 4 SBS's. Each one got about 15 minutes of attention so not much time spent on each. So did this XTR12 run, cycle as designed? Yes and no. The range didn't allow bird shot. No. 1 buck was the smallest allowed in shot size. I had brought several different loads to work with. 1200 FPS 1 1/8 oz load. 1300 FPS 1 1/8 oz load, 1265FPS 1 1/5 oz load, 1350 FPS 1 1/4 oz load. The order listed is the order of increased recoil. All were 7.5 shot loads. Was not allowed to use them per range rules so testing plan had to be altered. I started out with a 2" barrel extension installed as I wanted to see if the gun would cycle. I wanted to see if it would cycle with the lower powered shells listed. So I started with NO. 1 buck at 1200FPS 1 oz load using a new 5 round magazine. Cycled the first two rounds without issue. 3, 4, and 5 stove piped. Tried again and the stove piping continued. ? Changed the load to slugs. 1200FPS 1 oz managed recoil. Again it cycled the first two rounds and then stove piping returned. ? Then I noticed something. The gas block had moved towards the muzzle. Yours truely had not tightened the two screws securing the gas block enough. Dummy. The gas block moving away from the chamber was the issue. It has moved about 3/8 of an inch That shortened the piston travel and killed the effective drive needed to cycle the gun. I'd call it short stroking of the piston, poor gas port alignment, maybe some gas escaping. That ended the testing. I did not have the tools to relocate and secure the gas block with me. I did get some good data It will cycle with the lower powered shells with the 2" extension. This gas block with the larger diameter piston will work using the 5 ports mentioned earlier in this thread. Something to note is that the magazine used was new and quite stiff. Took a bit to get it loaded with 5 rounds. I will make a securing spacer between the gas block and the muzzle device so the gas block can't move away. I have some high strength high temp Delrin I'll use for the spacer. Super tough stuff. This means another round of testing (in the desert) using the shot loads mentioned earlier. More pics The spacer to retain the gas block will go in the space between the muzzle brake and the gas block. Removing a small amount of the bottom of the gas plug for a thicker walled spacer.   The gas block with piston fully forward towards the muzzle and the bare piston/rod. You'll notice two high temp O-rings on the piston. They cause some real noticable drag on the operation of the piston. I'm going to make a new piston/rod combo like this one without O-rings for final configuration with a slightly larger piston diameter. I'm still going to move the gas block about an inch closer to the chamber requiring a shorter piston/rod.   A look at the gas block port hole and the 5 ports it aligns with.   This is what the extension looks like. I have a selection I made, 1 inch increments up to 7 inches. This is the 2 inch. The knurled adapter is threaded M22x.75 thru, Saiga 12 and Vepr 12 threads. One adapter is 7/8-32 on one end, M22x.75 barrel end, for barrels with that external thread, as my 7" VR80 is configured. 
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I decided to make some more changes to the gas system and solve a couple of possible problems. These mods will put an end to cycling issues that could possibly be attributed to the gas system. The gas ports used for this first round of testing were centered over the 2.5" mark from the muzzle. So that gave me 2.5" of barrel dwell time. There were a bunch of ports not used that will eventually all will need to be welded closed. They were trapped under this new RAS gas block I switched to. No longer the case. I moved the gas block substantially closer to the chamber. I turned the barrel again to move the gas block. The ports will now sit at 3.945 from the muzzle. A 63% increase in dwell time. Ran out of time and didn't get the ports drilled. I solved three other issues with the making of a single part. A close fitting barrel sleeve. The sleeve covers the old existing gas ports and becomes part of the barrel assembly. I am concerned that distorsion of the barrel might occur during welding and the barrel getting hard at the area of welding. especially with so many ports to close up. That happens to Remington barrels. Weld up gas ports and they become quite hard. (heat treated). This recently happened with my 10" 1100. Welding of the old ports almost prevented the installation of the Rem Choke. Threading was quite tough. Back to the sleeve. It also solves the issue of the gas block moving towards the muzzle as the gas piston returns home. That happened last time out. it's possible I didn't tighten the two clamping screws tight enough but the block moved towards the muzzle which mis aligned the gas ports. The sleeve is precise in length and diameter. It fits tightly between the gas block and the muzzle brake, line to line fit. Of course the piston op rod was shortened even further. Removed 1.440 inches off the rod. I also started working on the handguard which needs a lot of modifications as this gas block just doesn't fit under it. First pic is of the my first run at the cycling problem. 2.5" of barrel after the gas ports. Second pic is new gas block location, about 1.5" closer to the chamber giving right at 4" of barrel after the ports. Nice increase in dwell time.  Gas block new location on the barrel without the sleeve and with the sleeve.   Better look at the sleeve, a nice problem solver. No welding the ports closed, ports covered, gas block secured in location.   A look at the ports being covered by the sleeve.  Still more to do with the gas block. Have to mill off a small amount of the plug on its bottom side. No clearance, plug hits the sleeve. About 1/16 of an inch will do it. There will be 4 gas ports on a .244 bolt circle. Starting with .080 holes but I'm leaving room to go up to .100 holes. That bolt circle will be centered at 3.945 inches from the muzzle. |
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The following pictures are from the RAS 12 that's in work. It's relevent to this build as well since its an AR10 upper. The obvious is high powered rifle builds too. I copied the info from that post to this for the upper receiver tool and laser/light combo tool sitting on top. I was looking for a tool to assist with setting up these 12ga uppers, drilling the ports accurately, etc. I came across this VISM Pro Series AR10 Upper R Receiver block. This tool solves most if not all of the needed support that will help reduce set-up time and accuracy. It securely locks down the AR10 uppers used on these 12ga AR uppers. It's a vise block that will very securley hold the upper vertical and solidly, especially for the drilling of the gas ports. Has notches for the uppers lugs, pins thru the take down and pivot holes. I would have liked the block that locks into the barrel extension but these 12ga's are all a bit different. Not a standard bolt interface. It will secure well enough for torquing the barrel nuts without twisting the upper. It's a great purchase for me because of the large number if AR12 platform shotguns I have. Also purchassed the AR15 version as well.       The laser/flashlight combo piece on top is a Votatu product, a P9L-G. Green laser and light combo. well made, finished nicely, high powered green laser, and a strong flashlight. All in a nicely packaged, well thought package. Looks good too! I have one of their red dots on a shotgun and it has been perfect and withstands the 12ga recoil.     |
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Update: Today I drilled the 4 new gas port holes at the correct tightly held location. Though my original thoughts were 4 holes, .080 diameter, I increased the size to .096. I'm looking for a faster response in piston movment and greater aceleration of the piston. This gas block uses (2) O-rings around the piston. It fits somewhat snugly in the gas block. This causes a drag I want to overcome. Or, I can remove the O-rings, no drag but less than ideal fit of this piston to wall clearance. If I lube those O-rings up, the piston slides with much less effort. The lube not being a good idea where burnt gasses and powder residue are headed. The higher pressure created by the gas block being much closer to the chamber has benefits. Higher pressure with slightly larger ports (more volume) would move the piston much quicker. It will also help solve another issue thats present. This gun has a substantially lightened bolt carrier. Not saying it's good or bad. When the gas drives the carrier rearward, it's lighter, acelerates quicker rearward actually speeding up the cycling. It also is easier to decelarate, easier for the buffer spring to stop it's rearward travel. Because of the less mass and bolt moving to battery, it's harder to strip the next round from the magazine, like using a lighter hammer when driving a nail. To overcome this, overgassing lets me use a stronger buffer spring, driving the bolt forward with greater force to strip the next round. This really comes into play with larger capacity magazines (10 round) stripping that next round out of a tight mag. A somewhat delicate balance. Even the ammo choice plays a part. All ammo is not the same diameter, nor do they have the same lubricity. Some shells just run better because of their size (diameter) and how slick the hull is. Thats where this set-up is headed. Trying to eliminate any finicky ammo issues. This XTR had a shortened buffer spring in it when purchassed. Feels like a standard AR15 spring, just shortened maybe a couple of inches. That means it won't see full compression, it will have less resistance at full travel. Gassing this gun. Just enough gas to operate would lend it to FTE's and FTL's I'm using the buffer spring it came with and a Kynshot 2.5" buffer to dampen the now designed over gassed system. If this shows any shortcomings, I'm ready to address that issue, what ever that issue is. A couple of pics for reference. Time to make another run to the range to see what I've got.  
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Originally Posted By diesel2: Thank you for this interesting write up. You are giving me ideas on enlarging my gas ports and the effects of dwell time. I assumed this was part of my issues with the JC Higgins I chopped. Now if it will just rain so I can go shoot! I'm with ya on the rain. We just started Monsoon season here in Phoenix. Humidity has jumped up, Now it's time for some storms with rain, and wind, and dust, oh my! It rains, the temp drops a bit. Humidity rises. Becomes less comfortable out side. High daily temps for July, with the lowest being 101° and the highest at 115° this year. Come on rain. Thanks for the coments. Always appreciated. I like to show what I'm doing. The process. My thoughts. I kinda go down an unchartered path with some of these SBS projects. I like short barreled semi auto gas guns and they always present issues to overcome. Thats a challange. One gun, one barrel, one gas system, just one of whatever to work with. I'm no expert, not an engineer. I doubt there is a school anywhere to teach this stuff. Just a lot of common sense, some logic, and a lot of machining experience. I hope those who have a project in mind might learn something, be incentivised to jump in. Showing the process, my approach, the testing, results achieved, how to keep moving forward. My philosophy? I try to answer questions, one might have, so they don't have to ask! It's information. On that JCH. If you have any questions, feel free to ask. Share some pictures of the working parts of that shotgun on your post. The bolt, the carrier, gas ports, piston, action rods, how it looks together and apart. where you want to go with it. Might help someone else with a JCH they don't know what to do with. I'm always interested in the workings of semi autos. |
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