You know what Fox... it's possible that I've misunderstood what you've stated.
Near round 70 I had a FTE leaving the PMC brass casing in the barrel. The casing was clean easily pushed out with a cleaning rod. Half of the rim of the casing was gone.
When you say "half the rim of the casing was gone"... are you saying the case head was separated and the case body remained in the chamber... or that a portion of the rim only was missing and the case head was more or less in tact... save for the missing portion of rim?
If it's the latter, and I've misunderstood your statement, then I apologize. That might very well be an entirely different issue... and perhaps closer to Quentin2 comments regarding cleanliness. It may still be a chamber or other issue and still may require returning the gun to Stag, but may be less of a safety concern than I initially imagined.