I’ll apologize ahead of time for the lengthy post but I feel like I need to give some background info.

I have been away from building, etc for a while and decided to build a 7.5” .223/5.56 pistol. I ordered all of my parts and assembled it. I went with the Maxim Defense pistol brace and a JP Enterprises Heavy buffer. For some reason the JP Buffers that Maxim sells with their brace/tube are proprietary to the Maxim brace (shorter than the other braces?). I built the lower and the upper. It was finished and I had not shot it yet due to work and only being able to work on my AR pistol at night in my garage after work.
I got a bug to build a .458 SOCOM 10.5” pistol upper and just swap uppers and use the same lower for both. I ordered all of the 458 upper parts. While waiting for the 458 parts to come in I went out and shot the .223 for the first time. The bcg wasn’t even being pushed back far enough to pick up another round. I had to manually rack another round in after each trigger pull. I immediately thought to myself that the heavy buffer was too heavy. So I went on Maxims website and ordered a JP Enterprises standard buffer spring. Well it came in today and I also finished the 458 upper. It is time to test again.

I installed the standard JP SCS and popped on the .223/5.56 upper. I was able to fire round after round after round. Nice. But after the last round was fired, the bolt locked back between the bolt lugs and the carrier its self. It didn’t lock back all the way like it normally should. I thought it was my reloads causing it. But I tried factory ammo as well. It kept locking back between the carrier and rear of the bolt lug.

I installed the 458 upper. I fired round after round but when empty, it locked back just like the .223 upper did. Between the rear of the bolt and carrier. Weird.

The bolt locks back when an empty magazine is installed and the charging handle is manually operated. Both uppers have a pistol length gas system.

Any ideas? What in the world is going on here? I really appreciate everyone here. This is the best forum for anything AR related.

my guess would be either the install or the combination of parts is not giving you enough gas to cycle completely, check to see which weight is on the jp and see if you can go lighter or get a lighter spring for it

Get a standard carbine buffer and spring and try that.

You buffer tube isn't screwed in too far is it?

Nope. Just enough to capture the buffer detent.

When you manually pull the bcg back with the charging handle where does the face of the bolt stop?  It should stop just short of the back edge of the ejection port being in front of it by 1/8" to 1/4".  But you are sounding undergassed or have too much weight in the buffer or sprung to heavy.

I'd look at the 223 upper gas system?  Holes lined up properly, nothing leaking gas?

Yep, misaligned gas port.

Have a great, gun carryin', Kenpo day

Clyde