300BlkTalk

Hello, I'm brand new to the 300 blackout cartridge. I've been lurking for a little bit while I acquired parts for my build. Well I finally got all the parts and took it to my gunsmith for assembly. I got the upper back today and I went to hand cycle a couple of the remington UMC FMJ rounds through it, just make sure the action function. at first none of the rounds would chamber at all. After adding some more CLP it started to chamber the rounds it still seems to not want to chamber rounds from time to time and it seems to be random.

here are the specs on the rifle:

Rainier 10.5" barrel w/m4 cuts
Rainier Gen I upper receiver with m4 cuts
YHM BCG w/ standard Charging handle
Larue Rail

The upper sits on a command arms pistol lower.

Magazines used were magpul pmags.

here is a pic of the issue and some parts on the upper:

Failure to feed/go into battery


Picture of barrel extension into upper.


Picture of BCG


I haven't had the chance to take it to the range to do a full on function test, hopefully get out there this weekend. If the gun fails to function properly I'm going to be really disappointed. I was looking forward to doing a 300 blk build and so far it's let me down but I'm not willing to give up on it yet. Help me diagnose whats going on please.

You should pull the charging handle all the way to the rear and release it. If you ride the bold forward I would expect it to stop before feeding a round out of the magazine.

I used both the bolt release and pulled the charging handle completely to the rear, it didn't seem to make a difference either way. The feeding issues happened with both.

Probably a weak action spring or drag on the carrier.

What spring does your pistol lower use?

I have a Palmetto State pistol lower parts kit in the gun with their standard spring and their H buffer.

You can check gas tube alignment. Take the bolt out of the carrier. With the upper inverted put the carrier about half way in. Tilt the muzzle down just enough for the carrier to slide forward, make sure there is minimal contact with the gas tube and the carrier goes all the way forward without the upper being tilted more.

After that try a new action spring.

Todd, that's a great troubleshooting tip. Thanks for sharing that with us.

Thanks, I'll give that a try.

I'm having the same kind of issue but mine is short stroking. Will fire the 1st round fine but bolt will not travel back far enough to pick up the 2nd round. Have change buffer & action spring and opened gas port up on barrel to .120 but still issues. Will extract one at a time but bolt does not lock back after each shot. Could the gas block port be to small and not allowing it enough gas to fully cycle?

I took my brand new CMMG 16" to the range for first time. Lower is a Spikes Multi-cal midlength with heavy buffer. I had FTF on every single round. Tired about 30 and got tired of that. Ammo is Remmington subsonic. Also eject failure on a couple rounds. I have not checked the gas system, but Im wondering if the buffer is the real issue since Spikes supplies this buffer with the Midlength specifically.
Any ideas?
thanks