First time messing around w/ 300 blk. I only shot one mag through the upper I put together and didn't have much time to mess with buffers and whatnot due to the range closing soon. I was having a situation where I would fire a shot and the case would not eject and a live round would feed up into the back of spent casing still in the chamber. Happened about every other round for a full magazine. 
Here is a parts breakdown if that helps. 

Vltor mur upper receiver
Ballistic Advantage 8.3" barrel and gas block. 
BCM BCG
Noveske SBR lower w/ Geissele SSA-E trigger
Pmags were used. I noticed that it was easy to push the front of the rounds down to nosedive, but I don't know if that would cause issues. 
The ammo used was Gemtech 147 grain supers
The buffer used was an H2
Spring was a sprinco blue spring. 

The first thing I can think of is changing the spring and buffer and adding an extractor o ring and maybe different mags and ammo. 

O-ring solved this problem for me, particularly for unsuppressed supersonic rounds.

For that barrel an H2 should work for everything, but I would not be surprised if you get some failure to lock back with a blue Sprinco and unsuppressed subsonics.


If you haven't tried Lancers yet, I'd highly recommend them for this application.

Ditto, Crane o-ring solved my FTE.

 Cool I'll have to try the o ring out first. Thanks 

Gas Block Alignment?

I thought about that as well, but the gas block uses a cross pin and set screws matched up for the barrel that came from Ballistic Advantage.

Did you get it running right?

check this. i had radical arms upper where the gas block "floated" enough that the gas pressure dropped enough that it would cycle badly and then not at all.

My radical firearms .300 upper, when I first fired it, wouldn't go into battery at first. Had to use the assist button for a full mag, then it was fine and dandy. Didn't bother me too much. My Bushmaster did the same thing when I bought it in 98. Both since then have been problem free. When I got my Radical Firearms 10.5" to build a sbr (built as a pistol 1st), it wouldn't eject a round after 2 mags. Basically had to charge it after each shot. No idea why at the time. Got home and took it apart to clean it. Noticed that the gas block moved a tick over 1/16" towards the muzzle. Pulled the handguard off and the set screws in the gas block were a bit loose. Pushed it back into position then took one set screw out and locktited it then did the other. No problems with the upper anymore other than had a mag feed issue with some old steel ones while playing with the binary trigger (pmags fixed that). Also locktited the set screws on the .300 upper also.