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Posted: 4/15/2012 9:29:46 AM EDT
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I would like to fabricate or retrofit a piston onto an AR10.
I have some piston ARs in 5.56 and 6.8 and although they are fun, they aren't 308. So what I am craving now is a new from scratch upper for my AR10 that will have the advantages of a short barrel (10-13") with the quick cycle of a piston gas system. I have a spare flat top upper and a spare 16" barrel with a carbine gas system. I can cut and thread the barrel to whatever length is necessary to support the piston system. I also have a few leftover piston systems from 5.56 projects past. They are .310" and .370" piston diameters. My FAL has a .430" and my M1A has a .500" piston. So generally speaking, what size piston would be needed in a 7.62mm AR type weapon? Anyone try the smaller pistons on a conversion yet? I can't be the only one here considering this mod, lets get something going on! |
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Can I assume that you're doing this "just to do it" and not because of a perceived "gap" in the market?
There are 7.62 piston designs already available on the commercial market, LWRCi and POF coming to mind first. Otherwise - what kind of machining capabilities do you have? Basic? Full machine shop? Honestly, I'd start with a good piston conversion system and backwards engineer it to fit your .308. For a "proof of concept" tester, you could probably go with a two-piece key shaped to interface with the op-rod for your existing .308 carrier. What are, exactly, the advantages you perceive from going with a 10-13" barrel in .308 that you feel are lacking in the 5.56MM or 6.8 SPC? Again, if you just want to do it to do it, that's no problem, I just want to make sure you're not wasting effort needlessly. ~Augee |
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Thanks a ton for the reply ~Augee!
Here is the deal, I don't want to invest in a new rifle, I have more MBRs than I really need already. I have a couple of Armalite AR10s and love the ergos, accuracy and aftermarket selection of parts for them. I have a few M1As and FALs too. I really dig the cycling of the action when firing my 13" OSW FAL. It is really fast, yet still adjustable and not too hard. Like any FAL, it is kinda heavy and not to comfy when chopping through a 30 round mag. Awesome fun, just not comfy. On the other hand, my 16" AR10 is super comfy when chopping through a 25 round mag. I just really don't appreciate the delayed cycling of the action when firing with the DI gas system. It seems like the action of firing and felt recoil is slightly delayed and drawn out over a longer period of time, than it is with my FAL or M1A. I have a spare upper and bbl, and I am willing to fabricate, buy or modify (within reason) whatever is necessary to try out a piston on an uber-compact AR10 build. I dig the Noveskie (sp) Lenodias and actually bought their gas tube to try out on a build (only if it is necessary), but is is still a gas (DI) system and I feel that it will not get me where I really want to be. Anyways, thanks again for your quick reply and I hope that this thread still will produce enough information to get this project started. T |
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So, just to add to the level of difficulty this project brings with it......I got this statement back from Adams Arms today:
"We will not be releasing anytime in the near future the .308. We do not have a time of if and when we release a .308 product." So...come on guys....almost THREEHUNDERDANDFIFTY views and no one is interested enough to add any additional information? I can't be the only one who wants to try out a piston on my 308 AR! |
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Isn't the M1A 7.62 NATO?
Seems like a good starting point. If I were doing it, my prototype would have the gas tube spigot and piston cylinder, as well as the piston and op rod, made in 2 pieces that screwed together so when (not if) the first piston is over or under gassed, only the piston and cylinder have to be made over and attached to the existing spigot and op rod. If you were in PA I'd tell you to bring it to my shop, I'd be game to try to make it work. Joe |
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I'd say the easiest method of making a piston AR in .308 would be upsizing the .223. Sounds stupid but you'll have a fun road ahead of you.
For op-rods, that is your biggest problem in DIY piston, go with using ejector pins from the injection mold machines. pcs is really good and cheap. If you have alignment issues make the flat head into a ball shape, sometimes with the flat heads they can have issues and pop the head off, make sure there is a receiving pocket surface on the carrier for the op rod to be located. Other issue is the carrier, if you go ahead and do a buffer that seats to prevent the piston from tilting you can reduce the stress on the lug on the carrier, ( I did a DIY buffer piston thing somewhere a year or so ago), for what you want try and do an attached lug with dovetail from the side or pin both need to be tool steel, otherwise i'd get another company's carrier and adjust op rod to fit it, End of op rod may mushroom so you may want a small chamfer. When the bolt is forward the oprod and the piston should have a very small about of play like .025'. For the piston, the diameter is the key, the larger the better and the more non problematic, like a barrel length comparison. .375 is good for .223, try to see what other .308 pistons are using approximately and start with that. The piston should just be a cylinder with standing ridges that can go in either way. don't need a return spring but you can add if you like. The Regulator that is a good one to use is the FAL style, the detent in the position selector should have a larger diameter but it works well, open up port on barrrel to a good size and let the regulator do its job, let the piston push just a little bit and you can start to bleed it. Short stroke tappet style separates the op rod and carrier very fast. gas blocks and piston should be 4140 or just about any steel. |
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Quoted:
I'd say the easiest method of making a piston AR in .308 would be upsizing the .223. If you have alignment issues make the flat head into a ball shape, sometimes with the flat heads they can have issues and pop the head off, make sure there is a receiving pocket surface on the carrier for the op rod to be located. otherwise i'd get another company's carrier and adjust op rod to fit it, End of op rod may mushroom so you may want a small chamfer. When the bolt is forward the oprod and the piston should have a very small about of play like .025'. For the piston, the diameter is the key, the larger the better and the more non problematic, like a barrel length comparison. .375 is good for .223, try to see what other .308 pistons are using approximately and start with that. Short stroke tappet style separates the op rod and carrier very fast. gas blocks and piston should be 4140 or just about any steel. Good Stuff ARMATAC As much as I love the FAL style, to attempt to implement it to regulate gas pressure on an over gassed 308 would be a bit too complicated. I would really like to keep it simple, with as low as a pressure as possible that will reliably unlock the bolt without excessively increasing it's rearward velocity. So, an over-sized AR .223 system sounds like a good starting point. I know what the M1a and FAL design uses for a bore and gas port location, does anyone know what the current 308 AR pistons use for diameters? |
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don't quite understand the comment on the regulator, you could just try getting the port sized right by slowly increasing port size, I would do this very last and make sure everything else is looking good. Even if it is short stroking keep shooting it because going the other way on ports is no bueno.
the FAL regulator is what POF uses I think. I have a shop and friends try to get me to make stuff all the time, maybe you could be satisfied with buying some parts already made and piecing a gun together to satisfy your diy instinct, I am not afraid to tackle anything, forums and the internet are great sources, I had never ever worked on cnc machines before and just jumped in and was able to rebuild spindles, axis wiring, anyone can do anything.... but making parts is like this, first you get your program together, then you have your first part ran, then you make some comp adjustments to make a better part if you have any sense, (remember coating thickness), then it takes you 10 parts to get it dead on and in 20 parts something else is slipping that makes you go back and have to sturdy up fixtures or something. It is just harder work than you would think. The difference between a mom and pop knocking out a bolt that looks good and works in the one rifle the shop had versus the bolts from FN, The first lowers I ever made seemed like the shit until one day on enough uppers I got some binds from the carrier, the extension tube was off center a few thousands, and I claim to be able to do a visual .001 relative difference(not really but..). |
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HERE is a shot of the POF system, looks like 223 tho:
I would like some 308 intel... |
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That does sound sorta doable.
I have been scouring used parts ads trying to find something and get some more ideas. Quoted:
You may be able to purchase a POF .308 bolt carrier group, then install a op rod and gas block from a FAL on to your AR10.... it might work. |
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Quoted:
That does sound sorta doable. I have been scouring used parts ads trying to find something and get some more ideas. Quoted:
You may be able to purchase a POF .308 bolt carrier group, then install a op rod and gas block from a FAL on to your AR10.... it might work. At least LWRC has bolt carriers for sale (or until they read this thread): http://www.lwrci.com/p-208-repr-bolt-carrier-assembly.aspx I'm not sure what the height differences would be between the gas block of the FAL and the carrier of the AR-10 but keep us posted, would love to see how it will work out. |
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Found this thread when I was researching to make my gaspiston 308. Finally got it together on Sunday. Haven't got to shoot it yet, so not 100% sure if it's sized right, but it has an adjusted gas block on it as part of the build and I believe it will work? Gasblock is 7075 aluminum and the gaspiston and rod and internal part of gas adjuster is 416SS. Used an A.A. carrier key for a gaspiston gun and their spring also to get the stroke of the piston sized right. Main part was it had to fit under the the freefloat tube and it just barely does,,, project is doable if one is interested,,, used small benchtop lathe and a minimill,,,
href=http://img402.imageshack.us/img402/2759/gasblock101.th.jpg" /> href=http://img23.imageshack.us/img23/6049/gasblock91.th.jpg" /> href=http://img831.imageshack.us/img831/7179/gasblock111.th.jpg" /> |
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Sorry, I suck at posting pictures,,, hopefully there're bigger this time??? I'm better at making things,,, hahaha
http://img191.imageshack.us/img191/2298/gasblock11.th.jpg http://img577.imageshack.us/img577/8373/gasblock10.th.jpg http://img600.imageshack.us/img600/8545/gasblock9.th.jpg |
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Quoted:
Sorry, I suck at posting pictures,,, hopefully there're bigger this time??? I'm better at making things,,, hahaha http://img191.imageshack.us/img191/2298/gasblock11.th.jpg http://img577.imageshack.us/img577/8373/gasblock10.th.jpg http://img600.imageshack.us/img600/8545/gasblock9.th.jpg Now you're talking TD!!! Could you pls send some pics to me as for some reason they are super tiny on my pc. [email protected] I have a mill and lathe and also the lil piece for the bolt conversion. |
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Quoted:
Quoted:
Sorry, I suck at posting pictures,,, hopefully there're bigger this time??? I'm better at making things,,, hahaha http://img191.imageshack.us/img191/2298/gasblock11.th.jpg http://img577.imageshack.us/img577/8373/gasblock10.th.jpg http://img600.imageshack.us/img600/8545/gasblock9.th.jpg Now you're talking TD!!! Could you pls send some pics to me as for some reason they are super tiny on my pc. [email protected] I have a mill and lathe and also the lil piece for the bolt conversion. +1 I would love to so some pics to, the ones on the computer are very small. |
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I will send them to you if this don't work. Think I spent more time trying to post decent pics,hahaha. I have no drawings or such, but could take measurements if needed,,,
http://img138.imageshack.us/img138/2298/gasblock11.th.jpg http://img546.imageshack.us/img546/8373/gasblock10.th.jpg http://img341.imageshack.us/img341/6049/gasblock91.th.jpg |
| Good news, bad news,,, nothing broke but the rifle did not fully cycle. I believe the gaspiston may have been too small in diameter resulting in not enough force being applied to the rod to fully stroke the bolt carrier. back to the drawing board. Going to make a larger diameter gaspiston and try again. The gaspiston internal diameter that I used was .257",,, not big enough I guess. I do not know what other piston diameters there are,,, |
| You are probably going to need larger, remember .375 for 5.56, so Pi/4*(.375^2) is the area. I would maybe start with that, again if you have a regulator you can just open up the port and let it decide the pressure. Aluminum gas blocks are maybe good for prototyping but you should go away from that. |
| Just my .02 cents, but I'd stay away from POF. Hogan used to make their parts and went into business for themselves. I got seriously screwed by POF, but ymmv. I'm curious to find out the results of the above poster who suggested using an Osprey Defense Piston System. I absolutely love my Osprey Piston that I have. |
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Good stuff guys. Here is where I am now.
FAL .430 M1A .496 AresDefense .374 Adams Arms .313 Using an Adams style piston block (gas block) will allow me to build up to a .437" piston. So.....Me thinks since my Fal shorty runs flawless with a .430 that that would be a good place to start. I know that I can build a system that will allow the use of components from Adams or from Ares, however I would like to standardize. The biggest problem I have encountered to date is the lack of straight line access on the AR10 chassis. Both the Ares and the Osprey use an off center approach which I think rules them out in an AR10 application. What I need to confirm is the exact distance from the centerline of an AR10 barrel to the center of the gas tube port on the receiver. I will need to match the distance perfectly in a newly designed piston block (gas block) to eliminate any chance of carrier tilt and or rod wear due to being off center. I have measured a distance of 0,8505" between the gas port center and the bbl center on an AR10 receiver. Can anyone confirm or deny this measurement? |
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Almost twothousand views and no solution yet???
An armalite SASS gasblock will provide a straight line shot to the gas port on the receiver. The SASS block is for an .875" bbl though, so a shim/spacer would be needed on a 750 bbl to seal the gas port. There appears to be enough meat on the SASS block, to allow it to be recut for an FAL type gas plug and sleeved on the receiver side for a piston/spring combo. Can a straight line gas block, with enough meat on it to be cut for FAL components, be had in .750" size????????? |
| Isn't an SKS piston around .310 in diameter? Also, wouldn't you need a smaller diameter piston rather than a larger diameter one for a short stroke piston to increase piston speed? I don't pretend to know the physics of it, but the short stroke pistons I know of have a have a smaller diameter head than a long stroke. (sks/ak, M1A/Garand) |
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I checked the sks, but I think I would really like to go with the FAL type design. Seems others are headed that way too.
I would really like to get my hands on a POF carrier and gas block for some measurements, or to use. POF refuses to sell any piston blocks or barrels or carriers to the end user. What a racket! |
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I purchased all of the gas piston components for a POF.
They are the same diameter and use the exact same piston plug ans the FAL. Go figure. Same as the FAL except that the POF and Hogan systems remove the spring from the op rod. The distance from the barrel centerline to the piston centerline is High, just like the AR10. I looks like the AR10 SASS gas block is also on the same high center and may be a host for a piston mod. |
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