Custom .300ACC Piston AR-15 SBR cycling issues.

[Thegeek]

I'll try to keep this brief. 11.3" barrel with an Adams Arms Piston kit. Why piston? Because I'm a lefty and shooting with a silencer on a DI rifle plain sucks. What I've put below is over the course of 2 years.

When I started this project, Adams didn't have a .300ACC kit, so I tried their 5.56 kit with a lightweight carrier. With .195gr supersonic rounds, it worked ok, but only at full gas. Step up to 220gr subsonic and function is pretty awful. It will eject the spent case, but won't pickup the next round. Contacted Adams who suggested drilling out the gas port to .125". Did that and no real improvement. At this point, my mind is stuck on the weight of the system not being driven back. I order a lightweight, flat wind buffer spring. It comes months later. Swap it out and at full gas, some of the 220gr will pickup the next round, but nothing locks it open on empty. Contact Adams again, who now informs me of a .300ACC kit. I order the parts that are different and install. The new gas block has 5 settings, and only on full gas (and suppressed) will it pick-up the next round reliably.

This is where I am today. I really want to put a video camera on it and watch it in slow motion. Part of me is wondering if I need a heavier buffer or carrier to increase momentum and overcome the spring. The current buffer is 3.0oz. I know this is an odd duck, but anyone have any ideas? I want it to function with the 220gr loads without the silencer.

 

[bwframe]

 

[T-DOGG]

What kind of ammo, reloads or factory stuff?

If it's not fully cycling, you want less reciprocating mass, not more. You already have the lightest factory buffer. Which exact buffer spring are you running?

The gas part of the equation. Pistol length system I hope? Before you drilled out the port, what size was it?

Anything binding when you operate the piston and BCG by hand? I assume cleaned and lubed properly.


Not enough gas to operate the piston is my initial thought.
- underpowered ammo
- dwell time
- port size
- install error partially covering gas port

 

[ROLEXrifleman]

For use with 220gr subsonic use a standard buffer weight (the 3oz you have) standard carbine spring and your lightweight carrier. The AA conversion will always be left in the full open position. If you’re using handloaded subsonics and even a good portion of factory loaded ammo in the 220gr WILL NOT FUNCTION without the can to produce the needed back pressure to cycle. It’s not the AA conversion that’s the issue, it’s subsonic 220gr that’s the issue. 220gr pills leave very little room for powder so in a 220gr load very little gas pressure is created. The suppressor helps pressurize the system and give enough to cycle these heavy loads.

.300bo is like a cat, it tells you what it’s gonna do, not the other way around. Hoping it’s gonna cycle with 220s and no can is losing proposition unless you go even lower on the buffer weight and more off the carrier but then you’ll have a gun that beats itself to death with any other load.

 

[T-DOGG]

For use with 220gr subsonic use a standard buffer weight (the 3oz you have) standard carbine spring and your lightweight carrier. The AA conversion will always be left in the full open position. If you’re using handloaded subsonics and even a good portion of factory loaded ammo in the 220gr WILL NOT FUNCTION without the can to produce the needed back pressure to cycle. It’s not the AA conversion that’s the issue, it’s subsonic 220gr that’s the issue. 220gr pills leave very little room for powder so in a 220gr load very little gas pressure is created. The suppressor helps pressurize the system and give enough to cycle these heavy loads.

.300bo is like a cat, it tells you what it’s gonna do, not the other way around. Hoping it’s gonna cycle with 220s and no can is losing proposition unless you go even lower on the buffer weight and more off the carrier but then you’ll have a gun that beats itself to death with any other load.

Excellent point. I really should have re-read the OP, specifically the last sentence before I tried to offer diag help on a system that's working.

220gr subs are made to be ran with a can. Without the can, run supers instead.

 

[Thegeek]

Fair enough. These are my loads. They're in the 1080-1090 FPS range. Using Hodgdon Lil'gun seems to give the best result. I tried with H4198 too and the performance was about the same. The lil'gun is more economic and meters a lot better out of my measure.

The spring is a Tubb's flat wind, light weight spring.

The barrel is a Wilson, and I don't know what the factory gas port size was. Drilled to .125 after Adams recommended and Wilson said it was safe to do so.

Clean and lubed. Manual cycling feels smooth and the piston system moves freely. It might have a total of 500 rounds through it, but only 100 on this new spring.

Overall, I think the rifle is still breaking in. I'm hopeful it'll get better over time. At least now I can shoot it without manually charging it every time. If I've done everything I can and my only option from here is giving up on the subsonics, so be it.

 

[Angrysauce]

What's the point of shooting subs unsuppressed?

300 Blk is designed to be suppressed when fired with subs. A pistol length adjustable gas system wide open, light weight carrier, and other non-standard parts aren't doing you any favors. You're trying to accomplish something that is objectively pointless and functionally impossible.
Look at every other 300 Blk on the market. They're designed to have just enough gas to run subs with a can and not to much gas to blow the gun apart with supers.
Throwing more money and "upgraded" parts at the issue isn't likely to fix your problem.

 

[Thegeek]

What I have now is all manufacturer recommended. Tubbs recommends the lighter spring for SBR .300 setups. The light weight carrier is what Adams recommended, and ultimately sold with the .300 kit. Essentially, I'm at "zero" with off the shelf parts. Being a custom build, off the shelf might not be correct, so I'm just asking for advise. If you're saying there's none to give and I'm chasing my tail, I can accept that.

Shooting subs unsuppressed I guess is a misleading objective. But, if I can get it to that point, it'll handle absolutely anything else by turning down the gas.

 

[AmmoManAaron]

Piston systems need both adequate gas pressure and adequate gas volume. Considering the cost of the other things you have tried, you might try a couple of other powders - could be a relatively affordable solution. I would try Accurate 1680 first and Accurate 5744 second. You don't specify what 220gr projectile you are currently trying to use. I recommend the 220gr Sierra MatchKing for your purpose. If you are trying to use a 220gr cast lead, that could be part of the problem as cast lead has a lower coefficient of friction and will produce less pressure at subsonic velocities than will the same weight jacketed bullet.

 

[Thegeek]

Lapua Secnar-L.