I bought new upper and lower .300 blk receivers and put them together, as a pistol.  I never attached a brace to the pistol.  I tried shooting it at the range for the first time and it wouldn't reliably cycle supersonic or subsonic rounds.  I was using an attached Surefire Socom .300 blk suppressor when attempting to shoot the subsonic rounds.  What was happening was the fresh rounds would get jammed between the magazine and the chamber as the weapon cycled.  I would manually load a round into the chamber and then it would jam again right after I fired it.  I tried cycling it using the charging handle and the rounds would occasionally jam as well.  I never limp wrist when I shoot and I've never had this problem with any other AR pistol, including my other PSA .300 blk pistol.  Has anybody else had this problem?

David

Very good question.  I'm using a Magpul Pmag .300 blk specific magazine and Ammo Inc. 220 gr Stealth ammo.

Edit:  These are the mags I'm using:  https://magpul.com/pmag-30-ar-300-b-gen-m3.html?mp_global_color=118

Yes.  I don’t think the problem is the gas port  The cartridges wouldn’t even chamber reliably when using the charging handle.

I just got this message from the PSA technician who was working on it:

"I switched out the piston and that tightened up the pressure in the gas valve chamber. to maximize pressure on the piston head. I also gave you a new sled, charging handle and spring. so the entire guts of the upper are completely new."

I'll test it again when I get it back.

THIS IS AWESOME SERVICE :)

Did the new parts fix the issue? I'm not sure how adjusting some of those things that were replaced would fix the issue you were having.
Like if the mag well/catch was out of spec etc... let us know if it was fixed because I plan on buying a 300 upper this week.
and thanks!

edited to add: I've had a few ammo related failure to cycle (low powder charge I suspect) with the Ammo Inc. 220 gr Stealth ammo.