Quoted:
what length barrel?
does the bolt lock back on the last round fired?
your feeding failures could be related to roughness between the carrier and the upper (lube properly) a poor fit between the carrier key and the gas tube (perform the 40 degree check outlined in the 23&P) or your feed ramps could be incomplete or mislocated (verify by eyeball). when the rounds failed to feed in what orientation did they stop?
FtEx are usually made worse by hotter (XM193 v UMC) ammo. I think your extractor is faulty and gets a better grip on the XM193 case compared to the others.
Tweak,
My barrel length is 16" and the bolt remains open on the last round. You mentioned a "40 degree check outlined in the 23&P, what is that and where can I find it? The feed ramps compaired to my bushmaster look normal. When the failure occurs the partially ejected case is out but is tilted slighly downwards while the fresh round is mostly stripped from the mag but not in the chamber , and is usually dented by the partially ejected round in the usually in the area where the case is necked down. When I remove the mag both the fired round both fall down out of the mag well.
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