I have a "CAR-15" 16 inched barreled upper: the barrel is stamped "SS" over 5.56 on the top of the barrel forward of the front sight, the upper is a A1 marked "DK" on the carrying handle. The upper work great on my old Colt SP1 lower. But when I put this upper on my old PWA lower serial #150xx, about once every third round I get the bolt digging in to the middle of the case while the bullet and case neck are just entering the chamber. I have measured the distance of various rounds with a know good round and find that in the PWA lower, the round is very slightl lower than in the Colt SP1 lower. Is this possibly the problem ? How should I try to correct this problem ? TIA

Edited to add: Adjusting the mag catch reduced this problem from every round to approx one in three.

What you are getting is the bolt not making it past the back on the round on ejection, grabbing the round at the top(front of the rim), and driving the nose of the bullet into the feed ramps as the bolt grinds into the top of the case.

You have short stroking!!!!!

The fist thing to do is check the gas system for leaks, then check the tail of the disconnector for signs that it is binding up on the hammer(wear mark just behind the hook), then move back into the carrier extension(buffer tube) for signs of binding. Also, remember to check the chamber for smoothness.  It may be a little rough and causing slight binding on ejection.

At ever step of the way clean the parts and apply a coat of CLP to them.  The new combo may just need to break in. By keeping the bearing surfaces of the rifle lubed, it allow the rifle to polish out any rough areas, due to anodizing or tool marks and the extra lube to over come them.

Hope this helps.

Dano523, I dont think is a gas leakage  problem because the upper works fine on the Colt lower. But I think you may have hit on the problem with the disconnector. I never thought to check that out. Thank You !!