AR Pistol: Buyers Remorse or Operator Error

[easilydistrac...]

I have a 10.5 inch AR Pistol in 300 blackout that I built from an 80% lower kit purchased from Thunder Tactical. I was unsure of the company but very pleased by the fit and finish of what I got considering the ~$340 price. But, I can't get this thing to run subs, and that's the whole reason I bought it. This is my first AR so I figured I'd check if there's something I'm not thinking of that might me causing this

The details
10.5 barrel (bear creek)
pistol length gas
standard carbine buffer spring/weight

I'm only interested in finding a hand load to work with this, though I did try a box of Rem subsonics to make sure it wasn't just my reloading skills causing the issue. Without a can the bolt barely moves, with the can they eject but no lock back and tend to jam if more than one round is in the mag.

15gr RL7 + 180gr sierra PH cycles fine, and accurate but dirty. Sub sonic charge is about 10.5 and ejects but not enough to pick up next round. I've tried 3 different cast boolits from 200-220gr and ~10.5 RL 7 with the same results.

Seems the gun is short stroking. I've check the gas block alignment, cleaned the chamber, stripped and lubed the BCG, gas key is tight, tube is clear, gas rings are good. There's a bit of leakage around the gas block but it doesn't seem like enough to cause this. If anything the gun ought to be over gassed.

Is it possible that the RL 7 is too slow a powder? I'm wondering if maybe the bolt is unlocking too soon? The only other thing I could think of is that the gun IS over gassed and the mag can't catch up. Adjustable gas block on the way whenever I can find a SA block in stock.

Thoughts? Is there something I'm not thinking of that could be robbing the BCG of energy?

[hpdrifter]

Well, I had carbine with carbine length gas system. The consensus here was misaligned gas block, leaking gas block, leaking gas system.

The purveyor of the 300 BO that I purchased put in a "300BO" spring. I saw them listed on the internet. Whether they are actually needed, I'll leave that to the experts. My carbine worked wonderfully after that. However, when I get more tools and doodads, I'm going to investigate the gas system.

Also, the consensus here, was the weaker spring was malarkey, more or less. With a pistol and pistol length gas system and from my very little knowledge, it seems like it ought to run perfectly without the weaker spring.

Just thought I'd give you some rhetoric until some more knowledgeable people helped you out. Good luck.

[BK7saum]

I think reloaded 7 is a bit too slow. Aa1680 is reported to be the best, but some use H4227, Lil gun and H110 also with good results.

[BK7saum]

I think if your 300 BO was a carbine system, you might have better luck with the slower powder. Did you chrono you R7 loads to check velocity. I'm curious what the velocities were.

[easilydistrac...]

I did but lost some of the data(mysterious phone delete). The cast loads were right around 1000fps with 10.5gr. With the jacked 180s 15gr got me 1400fps, 13gr around 1140, 12gr 1050 and 11.5gr 1000. Pretty close to the published data from Sierra.

[shortlegs]

iI agree with BK7saum Rl 7 is too slow for subs. i use lil gun and h110.

[TaylorS]

I run them well in mine with 8.2gr of imr4227 on 208gr a max think they ran about 1040fps been a while since ive run any


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[Moonie]

We have an SBR with a barrel that length, it shoots subs with RL-7 and AA1680 perfectly. I'd suspect either gas port alignment with the gas block or the hole being too small. We actually had to add a adjustable gas block to turn down the pressure hitting the bolt as it was over gassed with both of those powders.

[popper]

Check the buffer spring length, pistol tube might be short.

[dk17hmr]

H110/w296 will probably get it running.

[Texas by God]

Shouldn't it have a pistol buffer if there is such a thing?

[StratsMan]

My 300BO pistol gave me the same heartache with a cheap 7.5" bbl... I increased the dwell time by going to a quality 9.5" bbl... By then I had already added a low mass carrier and removed a weight from the buffer, so those remained. Now, it runs subs from 200 to 230 grain cast or jacketed with 10.8-11.2 grains of 1680, CFE BLK or Lovex D063 (ShootersWorld BlackOut)... It's very forgiving...

I'm convinced that the key change was the quality barrel (Ballistic Advantage)...

[DrewSeeg]

I agree with using 1680 or cfe blk... If you are not running a suppressor subs are still pretty dang loud. So I'm not sure it's worth the hassel. There are also "flaming pig" type muzzle devices that increase back pressure. Good luck

[Rcmaveric]

Thought the buffer and springs need to match the system? I would look into the gas port misalignment that's the first thing i look at. If that don't make it cycle i would look into the recoil spring and a buffer. Worse case scenario i wonder if your gas port is the correct diameter.

[easilydistrac...]

Thanks for the feedback guys. As far as I know pistol length gas is meant to be run with carbine buffer setup. Most guys actually run H2 buffers with a suppressor even with an adjustable gas block so I'm leaning havily towards this being a gas issue and not a buffer issue. I do plan on running these suppressed otherwise I agree the subs aren't worth the trouble. Gas port is, if anything, too big (1/8 drill bit fits loosely). I grabbed a pound of IMR 4227 and also ordered a new gas block. I've thoroughly checked alignment so I figure I'll swap blocks to see if maybe the block is out of spec. Probably be a few weeks till I have the time to reload/get out and shoot but I'll update when I do.

[popper]

Check the spring length yet? Mine had the same problem - the pistol tube I was given is shorter than carbine and really had to wrap itself up in the tube to get lock back. Rem subs would cycle - barely. 0.125 is OK for port, is a little large, IIRC carbine is 0.090.

[easilydistrac...]

I measured the spring and it was within spec, don't remember exact measurment. I haven't measured the tube but I will next time I'm tinkering.