[Metal god]

Is/was the upper new ? If the upper had been used/fired before and they used steel cased ammo .

Carbon could have built up in the chamber and caused the brass to stick to the chamber walls .

Steel case ammo does not expand and seal the chamber as well as brass does . When this happens the gases and other crap (carbon) can leak into the chamber and build up . If you do not clean your chamber really ,really well after using steel cased ammo the softer brass can stick to the chamber walls . When this happens you can rip the rim off of the cartridge or break the extractor .

Now that being said , This normally happen when you switch from steel case to brass and the chamber is still hot . ( ie one mag has steel the next brass )

I can see it happening with a cold bore if there is alot of buid up in the chamber . Im not sure how many rounds of steel you would have to put down range without cleaning for this to happen . I would think it would be many hundreds if not a thousand or so .

If it was new did any body give it the go , no go test and test fire it .