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I had a 5.45x39 upper for my AR, and the only ammo I could find for it was steel cased. After I had shot a magazine or two through it on each range trip, I had pretty frequent problems with FTE. The explanation that I got, which seems reasonable, is that the steel doesn't expand and conform to the chamber like brass does. This leaves room for blowback of residue into the chamber, around the steel case. After a while the residue is sufficient to keep the cartridge from extracting.