Adams Arms Piston Conversions

[barnbwt]

Could someone who has one of these verify the amount the piston is able to travel? By which I mean, how far back it can move when the carrier is to the rear before it comes to a stop against the upper receiver.

I'm using one in my AR-that's-not-an-AR build with one of those big chunky free-float tube barrel nuts, and the piston is short stroking as well as hammer-drilling a hole through the nut, slowly. I suspect the ~1/4" of travel it currently has is less than required, so it is both slamming into the soft aluminum barrel nut under force, as well as unable to cycle the bolt completely. I plan to drill out the barrel nut however much is needed, but I don't know how much is needed, lol.

Right now, when using the normal/full gas setting, I get nice consistent ejection, and the hammer resets, but it's not quite getting far enough back to snag the next round in the mag (let alone pick up enough momentum to shove it out the mag reliably)

Summary of the build for those interested;
-Freebie billet 80% lower ("phantom firearm")
-PSA cheapie LPK & stock/tube, from back when they were 25$ during the '13 Banic
-Carbine length standard weight buffer (going to an empty/light one did not fix the issue)
-Adams Arms rifle-length piston conversion
-Cheap Chinese free-float tube & barrel nut
-Homebrew 6.5 Grendel barrel from a chambered, unprofiled Sarco blank, .104" gas port (large)
-Lilja Grendel bolt & barrel extension (they just market them)
-Gibbz Arms side-charging upper (non-reciprocating handle type)
-At some point, I'll fabricate a telescoping buffer spring assembly to replace the sproing
-Nikon Prostaff 3-9x40 scope on extra tall cheap pic-rings
-Those steel ASC mags that score the hell out of the brass but are the only option
-123gr AMAX, Lapua brass, CCI primers, 28.1gr 8208, 2.25OAL

So yeah, total mutt of a gun with a random smattering of nice and incredibly cheap parts. Bears almost no functional resemblance to the AR at this point apart from the trigger group and bolt head, lol. Barrel shoots quite well for 50$, though the chamber neck is way too generous so it'll be getting replaced or set back & rechambered once I scrounge up a reamer. The thing is murder on steel; the FMJ PPU cheap stuff that got recalled recently (for being too fat at the case head & tending to tie up the gun in my case, while also operating at too-high a pressure) will put 1/8"x1/8" craters in AR500 at 150yds. Once I can get the gun cycling, I think it will be in the 1-2MOA range --I'm not really trying for groups at this time, as the malfunctions are too distracting.

TCB

 

[barnbwt]

No takers?

 

[Jackal]

Its probably not compatible with that barrel nut. Adams piston kit is sensitive to what barrel nut is used. Does Gibbz offer a piston guide insert (New Frontier Armory does and they manufacture the Gibbz unit)? It may be too loose where the piston enter the receiver.

 

[barnbwt]

It's not that it's loose, it just can't slide back enough to properly punt the carrier, I suspect. I assume that drilling the relevant hole in the barrel nut a bit deeper is what is needed, but I'm not sure how much travel is needed.

Or, I can keep shooting as-is, since the rod is hammering itself into the nut deeper with each shot, lol. 'Bout an 1/8" so far, and now ejection is becoming consistent, but no hammer reset, so it's getting close.

TCB

 

[barnbwt]

My Gibbz upper did come with a brass bushing specifically for piston conversions like these, which is installed.

TCB

 

[Jackal]

Sounds like the piston simply does not have enough rearward travel with that barrel nut. Either relieve some material or use a more compatible handguard.

 

[Arizona_Mike]

You seem to be answering your own question here about what the problem is. Surprised you are continuing to shoot under these conditions.

Go to AR15.com, the "Industry" forum and Adams Arms subforum and you are likely to get an answer within a day for your exact piston model and generation (there are several). Even If I were to tear my handguard off (which I really don't want to do as I'm using a Samson Evolution which is difficult to get tight enough to the point they don't move) there is nothing about my setup which would tell me the minimum travel that is needed, only that with a stock barrel nut I have enough.

How do you like your Gibbs Side Charger? I love everything about my American Spirit Arms except gas handling. No issue without a silencer but when shooting suppressed, I have to wear a bandanna to avoid powder burns on my lips!

Mike

 

[barnbwt]

With the bolt carrier back and the gas plug removed, you should be able to press the piston back with a pencil until it stops solidly against the receiver or barrel nut. Measure how deep the pencil sticks past the front of the gas block, and I'd be most appreciative. I have emailed AA directly, but I haven't heard back yet (I expect them to tell me to stop drinking)

I like the side charger a lot, actually, and I assure you there's no gassing (piston, obviously, and the rifle length should mitigate chamber blowby). The one issue is the handle detent is strong enough that it can stop the bolt going full forward, but I'm certain these crummy Grendel mags are a big part of that. Truly the Achilles Heel of the cartridge (well, that and the AR generally)

TCB

 

[lysanderxiii]

Could someone who has one of these verify the amount the piston is able to travel? By which I mean, how far back it can move when the carrier is to the rear before it comes to a stop against the upper receiver.

I'm using one in my AR-that's-not-an-AR build with one of those big chunky free-float tube barrel nuts, and the piston is short stroking as well as hammer-drilling a hole through the nut, slowly. I suspect the ~1/4" of travel it currently has is less than required, so it is both slamming into the soft aluminum barrel nut under force, as well as unable to cycle the bolt completely. I plan to drill out the barrel nut however much is needed, but I don't know how much is needed, lol.

Right now, when using the normal/full gas setting, I get nice consistent ejection, and the hammer resets, but it's not quite getting far enough back to snag the next round in the mag (let alone pick up enough momentum to shove it out the mag reliably)

Summary of the build for those interested;
-Freebie billet 80% lower ("phantom firearm")
-PSA cheapie LPK & stock/tube, from back when they were 25$ during the '13 Banic
-Carbine length standard weight buffer (going to an empty/light one did not fix the issue)
-Adams Arms rifle-length piston conversion
-Cheap Chinese free-float tube & barrel nut
-Homebrew 6.5 Grendel barrel from a chambered, unprofiled Sarco blank, .104" gas port (large)
-Lilja Grendel bolt & barrel extension (they just market them)
-Gibbz Arms side-charging upper (non-reciprocating handle type)
-At some point, I'll fabricate a telescoping buffer spring assembly to replace the sproing
-Nikon Prostaff 3-9x40 scope on extra tall cheap pic-rings
-Those steel ASC mags that score the hell out of the brass but are the only option
-123gr AMAX, Lapua brass, CCI primers, 28.1gr 8208, 2.25OAL

So yeah, total mutt of a gun with a random smattering of nice and incredibly cheap parts. Bears almost no functional resemblance to the AR at this point apart from the trigger group and bolt head, lol. Barrel shoots quite well for 50$, though the chamber neck is way too generous so it'll be getting replaced or set back & rechambered once I scrounge up a reamer. The thing is murder on steel; the FMJ PPU cheap stuff that got recalled recently (for being too fat at the case head & tending to tie up the gun in my case, while also operating at too-high a pressure) will put 1/8"x1/8" craters in AR500 at 150yds. Once I can get the gun cycling, I think it will be in the 1-2MOA range --I'm not really trying for groups at this time, as the malfunctions are too distracting.

TCB

Are you using the spring bushing? The spring bushing serves two purposes:

1) it provides a seat for the spring against the standard barrel nut, and
2) it provides a stop for the piston's rearward motion.

Not using the bushing would allow the spring to go solid during operation, severely reducing spring life, and possible reduced reliability.

.25 inch is way to short. The required travel is about 5/8 to 3/4 inch.

For exact measurements to correctly modify an aftermarket nut: measure a regular barrel nut and find where the front face is relative to the upper receiver. Now, relieve the aftermarket nut so the spring bushing sits at the same distance from the upper. Again, you really need to use the bushing.

 

[barnbwt]

Thank a ton, lysandexiii! My spring isn't bottoming out, the piston hits the nut before that, hence the short travel. The bushing won't fit under the handguard, but I'll figure out something that takes the abuse better once I relieve the nut & verify this allows for function. I'll bet that the impact force is a lot less when the piston can travel further, though, since the gas pressure will abate more.

TCB

 

[Jackal]

With the bolt carrier back and the gas plug removed, you should be able to press the piston back with a pencil until it stops solidly against the receiver or barrel nut. Measure how deep the pencil sticks past the front of the gas block, and I'd be most appreciative. I have emailed AA directly, but I haven't heard back yet (I expect them to tell me to stop drinking)

I like the side charger a lot, actually, and I assure you there's no gassing (piston, obviously, and the rifle length should mitigate chamber blowby). The one issue is the handle detent is strong enough that it can stop the bolt going full forward, but I'm certain these crummy Grendel mags are a big part of that. Truly the Achilles Heel of the cartridge (well, that and the AR generally)

TCB

You know, that detent is adjustable. Back out the detent screw slightly, its adjustable for that very reason.

 

[Acera]

Or, I can keep shooting as-is, since the rod is hammering itself into the nut deeper with each shot, lol. 'Bout an 1/8" so far, and now ejection is becoming consistent, but no hammer reset, so it's getting close.

TCB

I love that. Most shoot until it's broke, you going to shoot it until it's fixed.






.

 

[barnbwt]

You know, that detent is adjustable. Back out the detent screw slightly, its adjustable for that very reason.

Neat; good to know I have the option (I hadn't looked real closely for an adjustment, I was prepared to trim the spring a hair)

ETA: I now have no criticism whatsoever of the Gibbz upper, other than the fact the little gas-key tunnel cover dingus at the rear of the unit is spring-loaded up, as opposed to down (so unless you remember, you tend to put the gun together with this cover open and have to pop the rear pin again). It'd also have been cool if they'd built in some kind of latch or detent that keeps the bolt carrier held forward against its spring, so you don't accidentally slam the protruding carrier tail into your shiny new upper receiver when closing the upper to fix said rear cover dingus (there's a spring between the bolt head and carrier to help reduce wear on the smaller cam pin contact surface since the AR design has no anti-pre-engagement device like all proper autoloaders, so the carrier shifts backward unless the buffer is holding it forward)

TCB

 

[barnbwt]

I don't suppose one of ya'll has an AR sans handguards at the moment to check the distance from upper-receiver face to front face of the barrel nut? It certainly appears that I can easily carve out another 5/8" or so of travel for the piston, which then gives me a little extra room for the steel bushing (I actually made a new one that has a larger contact area which can still fit inside my handguard, where there's a hollow channel under each rail)

A few odd things did strike me as I was examining the piston/spring, though;
-That spring is hellishly strong considering what its job is; is it just returning the piston mid-cycle to avoid the returning bolt carrier, or is it actually trying to dampen the impact of the piston against the upper to avoid peening under that steel bushing? A larger bushing would not have that problem, if so, and a lighter spring could be used.
-That spring sure appears to bottom out just as the steel bushing contacts the first/smallest shoulder on the piston rod (I assume this is the hard-stop that limits the piston's rearward travel in practice). Maybe a few thousandths clearance between the coils, which seems like it'd be bad for spring life. Again, a lighter spring with smaller coils would have plenty of clearance.
-The large area of close-tolerance between the gas plug and gas block of this setup seems like a recipe for binding and stiff operation with even a tiny amount of fouling. Now, granted my gas port likely had some tiny burrs on it from first being drilled, but it seems that the small jacket flakes shaved off the first round blew up the gas port, the migrated around the gas plug, making for a very sticky extraction of this part for disassembly. Once the gold glitter was wiped off of the easily-cleaned nitrided parts, they slipped right back together as before. Fouling grooves or something would be a nice addition, and I can see why other piston designs use a forward-venting arrangement to valve the gas flow to the piston as opposed to obstruction, which tempts the high pressure gas to flow around the blockage along with fouling.

TCB

 

[boom boom]

Thank to the O/P for the original posting and thanks to those posting their knowledge above. I have been considering building an AA upper just to try it out and the posting above help with technical build considerations. Thanks for sharing.

 

[Jackal]

Thank to the O/P for the original posting and thanks to those posting their knowledge above. I have been considering building an AA upper just to try it out and the posting above help with technical build considerations. Thanks for sharing.

Superlative Arms piston system is better in every way, also allows for a much broader range of handguards to be used, take a look at them: http://suparms.com/collections/retrofit-piston-kits . Adams Arms is somewhat outdated and heavy at this point and more difficult to install/finicky.

 

[Arizona_Mike]

Superlative Arms piston system is better in every way, also allows for a much broader range of handguards to be used, take a look at them: http://suparms.com/collections/retrofit-piston-kits . Adams Arms is somewhat outdated and heavy at this point and more difficult to install/finicky.

Not sure it is "better in every way" but it is different. It is a bleed off regulator like an AK vs. a restriction type regulator. As someone who does a lot of suppressed shooting, I'm generally fond of restriction regulation.

I'm not sure this is any more low profile that AA's low profile version the XLP. In the case of my AA, I appreciate the fact that the gas block is long and held on with 3 set srews since it has to resist the pressure of the drive rod.

Mike

 

[Jackal]

Not sure it is "better in every way" but it is different. It is a bleed off regulator like an AK vs. a restriction type regulator. As someone who does a lot of suppressed shooting, I'm generally fond of restriction regulation.

I'm not sure this is any more low profile that AA's low profile version the XLP. In the case of my AA, I appreciate the fact that the gas block is long and held on with 3 set srews since it has to resist the pressure of the drive rod.

Mike

I shoot silenced as well (Griffin Recce 7). The bleed off actually reduces blowback more than restriction somehow. Its also quieter at the shooters ear, due to less ejection port pop with the pressure venting at the front of the rifle. "Better" was maybe an overstatement. Its just lighter, more adaptable with more handguards, quieter and less expensive than an Adams Arms XLP.

 

[lysanderxiii]

I don't suppose one of ya'll has an AR sans handguards at the moment to check the distance from upper-receiver face to front face of the barrel nut? It certainly appears that I can easily carve out another 5/8" or so of travel for the piston, which then gives me a little extra room for the steel bushing (I actually made a new one that has a larger contact area which can still fit inside my handguard, where there's a hollow channel under each rail)

A few odd things did strike me as I was examining the piston/spring, though;
-That spring is hellishly strong considering what its job is; is it just returning the piston mid-cycle to avoid the returning bolt carrier, or is it actually trying to dampen the impact of the piston against the upper to avoid peening under that steel bushing? A larger bushing would not have that problem, if so, and a lighter spring could be used.
-That spring sure appears to bottom out just as the steel bushing contacts the first/smallest shoulder on the piston rod (I assume this is the hard-stop that limits the piston's rearward travel in practice). Maybe a few thousandths clearance between the coils, which seems like it'd be bad for spring life. Again, a lighter spring with smaller coils would have plenty of clearance.
-The large area of close-tolerance between the gas plug and gas block of this setup seems like a recipe for binding and stiff operation with even a tiny amount of fouling. Now, granted my gas port likely had some tiny burrs on it from first being drilled, but it seems that the small jacket flakes shaved off the first round blew up the gas port, the migrated around the gas plug, making for a very sticky extraction of this part for disassembly. Once the gold glitter was wiped off of the easily-cleaned nitrided parts, they slipped right back together as before. Fouling grooves or something would be a nice addition, and I can see why other piston designs use a forward-venting arrangement to valve the gas flow to the piston as opposed to obstruction, which tempts the high pressure gas to flow around the blockage along with fouling.

TCB

.890 inch

The job of the spring is to return the piston to the forward position. It is nowhere near strong enough to stop of even slow down the piston/operating rod. There was 900 to 1000 pounds of force that started piston movement, a measly 13 pound spring isn't going to do much towards slowing or stopping the piston in 3/4 inch.

A few thousandths inch clearance is better than having the coils impact each other at the velocities involved. The spring needs to be heavy enough to return the piston before the carrier catches up to it, as hitting it would mess up the carrier motion. Granted, a longer spring would have been better, but they needed it to work in a pistol-length set-up, which required the short spring.

 

[barnbwt]

That's what I figured, but thought maybe it was cushioning parts like those crazy strong HK33 buffers.

My barrel nut is already a junkyard special so it'll be getting relieved at the 12 'o clock position for the spring/seat. It's also a convenient time to swap up to a quality Odin Works barrel, too.

Otherwise I'd be sorely tempted to simply turn the impact shoulder of the piston back as needed, and run a larger diameter spring around the piston rod. Oddly enough, a PPS43 return spring fits over the main rod perfectly against the tapered shoulder, AR70-style.

TCB

 

[barnbwt]

I drilled down .6" on the relevant through hole on the barrel nut and squared up the bottom of the pocket (it breaks out the side of the nut) with a 1/2" endmill. That brings the surface for the bushing to the same plane as the mil-spec barrel nut. I made up a larger strike bushing for the piston shoulder to hit against so it has a larger contact area on the aluminum barrel nut (I suppose the mil-spec steel ones would not have the same issue), spun the handguard on & secured it, then slid the piston/spring/bushing assembly down. With everything reinstalled, I now have about .7" of piston travel.

I also took the opportunity to install my new 20" Odin Works barrel, which seems a bit lacking in the fit & finish department for a 300$ barrel, to be honest. Bore & chamber were dirty, and they're using some kind of cheap barrel nut (vs. the Lilja-sourced on my homebrew barrel) that fits kind of loose in my upper's socket, and has a thinner barrel nut shoulder --so I had to shim it to get the handguard to clock right. My first shot through the barrel resulted in a moderately stuck shell, and revealed a fairly rough chamber (lots of striations and shiny rings all up & down the case and at the neck/shoulder corners; I expected scratches since the barrel nut lugs are new & still sharp, but not dozens of rings indicating a dull or over-driven reaming operation). The exterior was also kind of dissapointing; a very clean and fine lathe-turned finish (like 100tpi), but random swirl-scratches on top of it from someone cleaning it up with sandpaper or a wire brush, as opposed to a quality finish-ground (or polished) exterior. Something to keep in mind for anyone waiting months on an Odin Works product.

However, over the course of only five rounds I'd loaded up for test, I did get a very authoritative ejection on the full-gas setting, and the next (and last) round in the mag fed successfully into the chamber. Cycle still felt a bit slow & weak, which I'll chalk up to the rough chamber for the time being. It was definitely polishing out rapidly, but again, I shouldn't have to burn a bunch of expensive ammo breaking in a barrel, nor spin it up in a lathe with some 1000grit just to clean up their laziness.

Thanks again to the folks who supplied their experience & measurements; it saved me a lot of guess work on how to approach debugging this. I may still rig up the gas piston to use the full-length PPS43 spring simply because I think it looks cooler, and doesn't have nearly so high a spring constant, even though the force is about the same. I still have reservations about that tiny ring of metal on the piston being the strike face which arrests it, but I will just keep an eye on it & the bushing for peening, for the time being.

TCB

 

[barnbwt]

Superlative Arms piston system is better in every way

At the risk of thread-jacking a thread that has now fulfilled its purpose, I'm inclined to agree. The SA system is more like the VZ58, or every other clean sheet short-stroke system design I've seen (including the AR18 if you treat the op rod & piston head as a single unit). I honestly don't know why AA approached this the way they did by copying the antiquated SVT40 piston, I guess that piston blow-by tends to go forward or to the sides vs. back at the shooter, but with tubular handguards that isn't that big a deal at all. The lack of real gas turbulence grooves in the piston or gas tube struck me as odd; I'd have put some piston rings on there, and not just relied on a coarse lathe finish to create the turbulent gas seal. Not to mention their decision to only have three gas settings was rather short-sighted, though I gather the new offerings have five or so settings. Having the gas 'plug' be a hollow cylinder that the piston head fits into would allow the gas block to be an unstressed part, much smaller & possibly even aluminum. I haven't found a good description of exactly how the SA piston vent valve works, but the obstruction-type of the AA does seem to attract blowby fouling that makes it really hard to turn or remove if a few shots are fired with the gas fully cut off. Forward venting is better all-around; if worried about gas vent noise, make a silencer with low back-pressure design and run a vent tube into the can from the gas block.

I won't hold the dumb pic-rail block against them; for some reason lots of people thought they were cool at the time, and they aren't making them that way any more which shaves a ton of weight; I can always grind it down if I really care that much. Now I'm all tempted to try making my own system...

TCB

 

[barnbwt]

I'm not sure this is any more low profile that AA's low profile version the XLP. In the case of my AA, I appreciate the fact that the gas block is long and held on with 3 set srews since it has to resist the pressure of the drive rod.

I'm sure the blocks are almost identical (well, mine is huge because it's a clamp-on type, and has a rail, and is 2" long, and is solid steel). Physics dictates the two pistons must have similar driving area for the gas to act on to cycle the gun similarly, and since both pistons slide out the front, the gas block is constrained to similar geometry.

TCB

 

[barnbwt]

Bit of a followup, gun runs reliably on the full-gas setting, but is throwing rounds far to the right/rear (somehow not quite hitting my shoulder) so I think the gas is still a little weak. But hey, I'm shooting reloads so that's not necessarily unexpected. The barrel was changed out to an Odin Works piece I got a fairly good deal on, which appears just as accurate in my incapable/unserious hands during test fire, but which doesn't blow the necks out to 30-caliber every time. Hopefully the cases will hold up better & resize a tad easier now

I'm still liking the cartridge; pretty cool how HPBT dents AR500 plates at 250 yards. I've had to move my steel-shooting further back since it was getting repetitive

TCB

 

[lysanderxiii]

Why AA did the design the way they did? Cost, and ease of disassembly.

The whole piston, gasplug and spring come out through the hole in the gas block (original design), which is useful if you have a free-float rail. The later ultra-low profile versions are impossible to clean if you have a free-float, unless you want to disassemble the whole front end. But were necessitated by not wanting to redesign everything.