Easy one, and flood around the base of the carrier key to top of carrier, hold the bolt inwards, then blast 100lbs of compressed air in to the front of the key.  What your going to find, is key is leaking badly to base of carrier.

Options are to either send the carrier back to be replace, or will need to resolve the leaking key problem. As for why the problem, there was a bolt torque increase spec for the carrier keys way back, but that was when the USGI spec called out for stronger bolts.  So manaufactors are not using the stronger bolt, but torque'g to these specs, and just a mater of time until the weaker bolts do start to stretch.  Also, some are not using the correct sealing compound between the two, which can also blow out cause the problem  as well.


And to prove the point, use the old bolt on the new carrier that is working/will see the problem to be the carrier is self (leaking key base to top of carrier).


Also to point out, leaking key  on B/C is a lack of gas problem, not too much gas instead, so adjustable gas block is not going to solve the problem, since it can only reduce the amount of gas to B/C, not increase the pressure.

Would pull the bolts again, do a bit of lapping on the bottom of key and top of carrier, light coat of Loctite works well for the gasket sealer between the two, key bolts at 27inlbs(blow through the key to remove any loctite that got into the gas passages, , and just remember to stake the sides of the key surface, to peen the softer key metal into the side spines of the hardened bolt heads to hold them in place; not try to peen the  hardened bolts head tops, since they are hardened and will just cause bolt damage isntead.

As for if you do see any old signs of the key being peened, then just replace the key bolts, since chances they where over torqued, and what caused to the stretch/become loose.

Hence torque value of the key bolts did change when stronger bolts became the replacements, but on some non military carriers, older weaker bolts still being used, torqued to the new higher values or 37 inch lbs, and what cause the bolt stretch from the start  problem.