I've had four Bushmaster AR carbines come in the shop over the last week for failure to extract. They were all in the BK5------ serial prefix range. In all four guns there was a rust ring about 1/8-1/4 inch wide just behind the shoulder in the chamber. Severity ranged from light to moderate and correction ranged from 00 steel wool or 600 grit silicon carbide paper on a split dowel.

FYI,

Bob