300 BLK AR Pistol issue

Throwing this out there. Maybe someone has done this before.

About a year ago, maybe longer I built an AR pistol from an 80% lower and a stripped upper. Went together just fine. Nine inch barrel. All good stuff. No cheap parts.

All is well and good until I get to the range. It won’t cycle (eject) properly. There doesn’t seem to be any interferences inside the gun and I see no apparent problems with the BCG. I suspect the issue with with the buffer and buffer spring. Either I bought the wrong parts or the wrong parts were sent to me. Since the spring and buffer have no markings, I have no way to tell.

Thoughts on this? And if you have built an AR pistol in 300 BLK… what buffer and buffer spring did you use?

Only way to tell on buffer is to either weigh it or take it apart to find what is inside (careful some might be powder). Spring is really no way to tell besides length (rifle vs carbine). Maybe a sprinco yellow with a carbine buffer, could also pull your buffer apart and empty the weights and try shooting it again.

Carbine - 3oz
H - 3.8oz
H2 - 4.6-4.7oz
H3 - 5.0-5.4oz

A 9" barrel is pretty short, with a short dwell time, did you happen to measure the gas port size and make sure its lined up properly?

So if you load a single round it doesn’t eject and lock the bolt back? Have you tried different ammo brands, subsonic vs standard velocity? Just watched a reloading vid for 300blk, he was loading subs and it would not cycle his ar either, untill he used non subs and factory ammo.

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I’ve never messed with 300blkout but did you seal the gas tube in the block and the block on the barrel? I’ve had leaks cause cycling issues.


I did not seal it. The gas block seemed tight and fit properly but I will double check that.

I have debated getting an adjustable gas block to see if that makes any difference


Subs predictably failed to eject. Standard loads were intermittent.

My gut is telling me it’s the buffer but I can’t rule out a gas issue. Thx for the info.

I’m going to dismantle the entire gun and try to figure out what I did wrong.


I have a 8" and it runs everything fine with no issues at all. The only thing that upgraded Im running is an Armaspec silent capture spring. Standard gas block. One thing you might check and might have already is make sure the gas rings on the bolt are staggered. Also make sure that the bolt is sliding back in to the tube smoothly. Please keep us informed on what you find.


I have an 8” with a Strike Ind flat wire spring, and a one piece gas ring. Every new one I assemble I dry cycle about 100 times then clean and lube before going to the range. Haven’t had any problems with supersonic. Including 194gr @ 1450fps.


I was up early this am and fiddling around with the pistol, getting ready to take it back apart. I had put it aside and it’s been sitting idle for a while. I noticed that the bolt needed an extra push to get into battery. Hardly any pressure at all was needed. I don’t think it’s a bolt face/barrel mismatch (one or the other out of tolerance)because it takes so little effort. However… the bolt and barrel are not a matched set. Bought separately.

Couple of thoughts…

  1. It’s a break-in thing that may go away if I put a few hundred rounds thru it.
  2. Back to the buffer. If the spring isn’t strong enough to push the bolt back into battery, I may have the wrong spring or the buffer is hanging up. The buffer seems to be OK but I’m not ruling anything out just yet.

I’m going to take it all apart today and go over everything. I’m ordering an adjustable gas block, switching out the one I used. Also a new spring and buffer. Not expensive, so I’ll give that a try.

My game plan all along was to shoot subsonic and suppressed in this pistol.

Related Sidebar: Unfortunately my AAC can suffered a baffle strike (WTF) and the new AAC will not fix it. The original AAC was a Remington subsidiary. The new AAC doesn’t give a rats ass about former customers. Been round and round with them. They said buy a new can. I eventually will, but not from them.


That really doesn’t make any difference. Those rings are constantly moving during operation.
An adjustable gas block can be helpful when running a short barrel 300 with variable loads with/without a suppressor.


My friends pistol only cycled subs while suppressing. Supers were fine unsuppessed. He thought he wanted an adjustable gas block, but after the realization he had no need to run subs loud, he just cans the sub loads and all is right.


Exactly what I do! I’ve never run mine without a can, 8.5” barrel with Rugged Micro 30.

Its either a gas issue or buffer/spring. An adjustable gas block won’t help as they reduce gas not increase it. If supressed then it will help if over gassed. Since your taking it apart, pull the gas block off and check the gas port on the barrel, in the gas block and also in the gas tube.

As for bolt and barrel not lining up you will see damage on the bolt face on the locking lugs.

If you stand the bcg on the bolt with the bolt out does the bolt stay out or does it slide down to the lock position? That tests if the gas rings are good or worn.

Can also check with a piece or weed wacker string or a aersol can spray tube and stick it down the gas key to see if the gas key is properly aligned (take bolt out and look down the bore).

Also what barrel and bcg are you using?


Bolt stays out.

Barrel is Ballistic Advantage. BCG is MAS Defense.

I’m going to pull the gas block off tomorrow and try to determine if it’s misaligned or restricted in some way.

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I can tell you the roll pin wizard is a nice too. Maybe the genie is too. Seen them in the lgs here too….

Or, it’s so simple of a design, I’m sure you could replicate it easily.


Did you space the gas block .030" from the shoulder?

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I was pretty observant and careful about lining it up, but as I recall when I put the gas block on it aligned perfectly with the machining on the barrel. But… I may have go it wrong. I’ve built a few ARs and never had this issue. First time for everything. In this case, first time working with 300 BLK. Process of elimination. I think we are all agreeing its either a spring/buffer issue or gas related.

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It’s not a matter of machining. If you didn’t space it .030 inches from the shoulder, it’s probably under gassed. A lot of AR manufacturers simply make the gas port larger to compensate for lack of understanding, but a lot don’t - they follow the mil-spec, which means a .030 gap between the gas block and shoulder, unless a handguard retainer ring is present. It’s easy to gap the gas block .030 inches and try it. Hell, a credit card gets you in the ballpark. Plus it costs you nothing but 5 minutes and 2 or 3 rounds.


Yep. I am going to bet gas block. When the AR pistol craze started I built one in 5.56 with a 9" barrel. I never could get it to run right. I tried everything. Every buffer weight and spring made. Three different BCGs. Adjustable gas block. Custom reloads. I gave up and converted to a 300 BLK. Well first time out it FTE. But I did have the adjustable gas block on it. A few tweaks on the GB and BINGO! Now I will say this. I adjusted for subsonic rounds with a depressor. Hot rounds with the depressor had cycling problems. Again a tweak or two and ran like a scolded dog. The short barrels seem to be picky about the amount of gas to the BCG and can run to slow or to fast with loads at the extremes.
PS all my builds now have adjustable gas blocks. They solve a lot of problems.



Agree. That’s exactly the path I took. 5.56 pistols make good flamethrowers but are not suitable for a short barrel. So I shifted to 300 BLK. Better, but still a challenge to get dialed in.

Overall, I’ve concluded that rifle caliber pistols are a big waste of time and money. It’s a solution looking for a problem. The fundamental purpose of a pistol is short range shooting. Rifle ammo and operating systems are really not designed for that. I’ve gravitated to PCCs for intermediate range shooting.

Stating the obvious, pistols are for short range, PCCs in the middle and rifles for long range. Guns like the Stribog, Sub 2000, Ruger PCC and Scorpion and a few others fit that middle ground perfectly. The Stribog is technically a pistol and the Scorpion comes in both pistol and carbine.

In the PCC category I also prefer folders and/or takedowns. I’ll go out on a limb (again, obvious) and say that anything beyond 50 yards is best handed by a rifle/carbine. Unless one is highly skilled with firearms, sending a round downrange using a pistol beyond 50 yards makes any degree of accuracy a challenge. Unless you are shooting from a bench with the pistol in a Ransom Rest.

I have an AK pistol (7.62x39) and one in 9mm. The 9mm is superior. The 7.62 pistol was fun for about ten minutes. It spews a lot of lead into the air quickly, but controlling it for any degree of accuracy is futile. Again… I asked myself what is it’s purpose and came up with nothing. For combat scenarios in a urban environment, maybe trench warfare… sure. But that’s an unlikely scenario. It’s nice to have in reserve in case of a zombie apocalypse though.

I’m going to get this 300 BLK pistol working then it will likely end up in the back of the safe.


I have not tried my 300BLK pistol with anything lighter than 147gr and H110. I have also had great results with Fiocchi & Freedom with 150/147s. I have built a pile of 7.5” 300s for people. I do the mathematical analysis, then find someone in need of coyote ventilation and test my results. With heavy bullets, they definitely have the advantage out to about 150yds. Far enough out here in the plains for anything I hunt.