Finally got a chance to take a new pistol build out to the range.  Very accurate and pleasant to shoot using a krinkov flash suppressor.  No fireball.  I was surprised due to all the negative stories about shooting an AR pistol with A2 flash suppressor.

Only problem is, the bolt wont stay locked back on last round. I tried three different Pmags, as well as a DH mag and a steel mag, all the same result:  dry fire, the bolt will lock back with an empty mag.  Live fire, the bolt does not lock back on the empty mag.

It's a 10.5" PSA barrel with a carbine gas system, using a pistol buffer/spring in a pistol receiver extension.  Using FA premium BCG from PSA.  Well lubed.  Brass kicks out like it's supposed too.  

Kinda confused....

What ammunition are you using?

Some Perfecta as well as LC and PMC.

Did you build the upper, or factory built by PSA?

Reason I ask is because most frequent cause is misaligned or loose gas block -- running at very edge of enough gas to operate, meaning carrier moves back enough to pick up next round, but not quite far enough for bolt to catch on the stop.

Also ensure gas key is tight, perhaps the second most often cause.

- OS

edit: I see I mainly echoed Josh, we were typing at same time. :)

Thanks guys!

I built the upper myself.  The gas block is aligned properly, and I just checked again, not loose.  Im lookin at the buffer/spring as a possible culprit.  The pistol buffer kit I installed was one from Opticsplanet Tacfire AR15 Pistol Buffer Tube Kit.  I  was trying to finish the pistol build before the election so I bought what was in stock at the moment.  

The spring that came with the pistol receiver extension only measured 10 3/8" long. I replaced it with a carbine spring that measured 11.5" long. I also replaced the buffer that came with the pistol receiver with the buffer that came with the carbine receiver extension.  What is funny is the shorter spring is heavier than the longer spring.

Most likely wont be until this weekend to see what those changes bring.

I had this issue with a rifle, and it was an upper buttstock screw that was too long.  Don't know if pistol buffer tubes have stock screws, just offering possibilities.