Better guess is the firing pin collar is hanging up on the hammer once in a while.

Also, why is the charging handle getting stuck mid steam of the mag?  It should be locked back on the receiver, or due you mean that if go to try and use it, the charging handle will not pull back cleanly? If the later is the case, then confrim that the rig is not low feeding from the mag (bullet tip striking the upper receiver just below the barrel extension feed ramps, and the rig is try to feed a bent round into the chamber.

Failure to lock up comes down to the B/C movement.

Out the gate, is the rig full stroking, hence will lock the bolt back on the catch with only a single loaded round in the mag fired?  With the standard buffer and the violent action cycle of the very short barrels, it may take a H-3 buffer to control the action a great deal better to prevent over function if the rig is short stroking.


If the bolt is locking back on the catch (full stroking and a heaver buffer not needed), then ride the CH forward by hand on a full empty charge and see if there is binding as the B/C goes forward empty. As stated, could be the FP collar catching on the hammer, or even burs on the recoils spring catching down the tube causing a less then ideal forward smooth stroke.  Granted that the bolt may not fully lock up all the way, the bolt lugs should enter the barrel extension and at least start to rotate.

If the action silky smooth with the bolt not being jerky as it goes forward, then it will come down to the problems with the round being stripped out of the mag at that point.  Could be as simple as the mag catch not tightened enough to cause low feed, to even burs on the mags not allowing a clean round strip and feed.

Bottom line is the rig has to full stroke from the beginning to get the bolt face all the way back before it forward journey, then the return stroke needs to be smooth with full power as well for the needed round strip/feed, and bolt lock up.

Bolt face almost about to touch the barrel extension when the binding is happening?

The first thing to weed out is the confirm that the gas tube is aligned with the carrier key.
With the bolt removed from the carrier, insert just the carrier into the upper receiver and look at the alignment of the tube as it enters the key.  If needed, then adjust the tube above the barrel for the correct index.

If that is not the problem, the chances are it's the hammer catching the FP collar.
Here, start off by confirming that the collar of the FP is even with, or just slightly below the back of the carrier.  If needed, chuck the FP in a drill (back end of the FP in the chuck) and spin file the OD of the collar until it is even of just slightly below the back edge of the carrier.

On the hammer, if a L cut style, check the top of the bottom L edge for signs there as well.  Chances are the FP collar correction  should solve the problem, but if not, you may need to round the bottom L cut to the top of hammer contact part edge.