When you put a new barrel on, you should check it for proper headspace so you do not blow up the barrel.  If you are using the same lower receiver assembly as you had on the 16", and the 16" had a lot of rounds on it, then you should consider changing out the action spring (buffer spring).  Also keep in mind that on the short barrel, you are not burning up all the powder, so you will see more fouling, of which fouling will cause parts to wear quicker.  I would recommend changing out the gas rings, and the firing pin retaining pin.      

CY6  
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123

Best guess, gas rings are toasted since the Carrier is out of spec.  Here, it would be the larger ID to small ID step in the carrier milled too far inward, and the rings can escape from the smaller channel to the larger channel as the bolt unlocks.

As for FP retainer, ditto on the chances you have a L cut hammer, and the top of hammer contact edge is catching the stop collar as the B/C moves forward home.
Note, when you install the FP retainer, eyelet opening front to back so the opening is in line with the front to back with the B/C, which orientates the legs stronger front to back as well.


The quick check is to make sure that the FP collar is not proud of the back of carrier when the FP is seated forward, and the hammer top of contact pad is not a sharp edge.

In regards to the FP collar, it can be reduced in OD but just chucking the back end of the FP in a drill and spin filing down the collar OD.  In the case of the hammer, once out of the rifle, sand paper on a dropsical stick or a diamond file to slightly radius top of hammer contact edge.
Note, you are cleaning up a mess that is caused by the trigger being too short from the pin channel to the front of the trigger, hence hammer not retained low enough during trigger retention.