300 won't lock back

[Howler]

Here is the deal. I've put together a 300 blackout and it fails to lock back on an empty mag. However, the bolt will lock back on a empty mag using the charging handle. The brass was landing at about the 5 o'clock position at best. This was the first time it's been shot so I only loaded one round at a time to do a function check. My first thought was a lack of pressure at the gas block. I blindly moved the block around at the range with no luck. This is the first time using an adjustable gas block and the adjustment process seemed to be simple so I don't think it's the block itself.

I still think it's the gas ports not lined up. So here's what I've done. I removed the block and marked the barrel gas port with a Sharpie. I backed the adjuster screw on the gas block out to give the system the full amount of gas. Also added some thread locker to seal the screw threads. It seems that I get the best air flow with the gas block about .010 of an inch from the barrel shoulder. Why? I don't know. Maybe someone here can tell me.

My parts are: Mas Defense 16" barrel (pistol length gas system), BTE micro adjustable gas block, using PNW Arms 125gr Nosler Ballistic tip ammo. Is there anything I'm missing? I don't like taking things apart over and over.

[LawEnforcementSalesTN]

Mike, most barrels are machined for the shoulder to accept the old CAR handguard retainers.  While most GBs may still function with it butted up against, the best is slightly forward as you're finding.. it sounds close need to get the shells to eject around the 2-3 o-clock position .. What buffer are you using?

 

Usually the bleed from the pressure will create a little ring around the gas port hole... and you can start to see about where the GB's port is in relation ..

 

I'd try to move the block slightly forward a bit more... put a couple of rounds in the mag and be sure it's cycling.  Check your gas rings and be sure they haven't wandered around and lined up and/or broke, etc.. check your carrier key and be sure it's tight.

 

If everything is new I'd run up to 50 rounds through to see if anything simply loosens up.. you're welcome to come use by buffer, spring, BCG, etc... if you don't have a spare but that's the next step just start swapping out what you can. 

 

Best of luck

[Lumber_Jack]

Definatley gas issue needs to be checked and resolved or ruled out.

What buffer weight are you using.

I just saw you were using a pistol gas system. Make sure the gas port or clear and not obstructed. Them align the block. You should be able to get plenty of gas with a pistol system.

I run my block about half way closed even without a suppressor. Edited February 16, 2015 by Lumber_Jack

[Dolomite_supafly]

I used a 16" barrel with a pistol length gas system for a very, very long time and it is the best setup as far as reliability goes. Check the port size and if it is not .088" I would drill it to that and use the adjustable gas block to tune the gas signal. With an .088" port you should easily get subsonic ammunition to cycle as well as supersonic but I would meter the supersonic loads to prevent over gassing.

 

The distance from the gas block to the shoulder that Mike is talking about is 10 sheets of standard paper if you do not have feeler gauges. If you do then the distance the gas block should be from the shoulder is .040".

[Howler]

Now that you guys have mentioned it I remember someone talking about the handguard retainer. Totally makes since. The buffer is a standard carbine buffer. The gas port in the barrel is clear and the block and tube are clear. The gas rings were in the proper position and the key wasn't loose. I'm sure the block wasn't in the proper place. The first chance I get I'll check the barrel port size. Never thought about the barrel port being too small. On the up side the group size at 50 yards was better than I expected it to be. I had a couple holes touch. Thanks for the help guys.

[gomer pyle]

I had the same problem. Mine ended up being the tube and block not lining up. I had to put them together and drill the tube to line it up.

[Ron Padilla]

One of the things I keep on my key ring is a spark plug feeler gauge (dont ask my why) I just do and it comes in handy for several things.  Paper can become depressed if squished to tight, but the feeler gauge wont, you can pick one cheap at any parts store insert it bettween the block and the and barrel tighten the block down then you know it is set at the gap you want JMO good luck.