What is the reason that "missing chuck" from the receiver behind the bolt's left lug needs to be repaired? The bolt's left lug is UNSUPPORTED in a "normal" carbine throughout the firing process. I'm not sure why it's even there except perhaps to provide general guidance for bolt travel when unlocked.

FWIW, at least one of my carbines has a similar "missing chunk" from that corner of the receiver as it came to me directly from the CMPicon. Presumeably they inspected it and I thought it was in fact "normal" at one time (my drawings aren't particularly clear in that area) but I have since been shown over on the CMP boards that it was due to breakage as it is obviously different from "new". But this carbine has fired thousands of rounds full auto in my possession without so much as a hickup. So I ceased worrying about it.

Knock on wood