Quote History Quoted:
Somebody likely twisted it at one point in time during a barrel install/removal.
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I have never managed to do this, even while breaking two barrel wrenches.
My F5 can cannon will only accept one out of a half dozen carriers I own. It is a cheap PSA bcg that I normally run in my .300AAC SBR. I even put a label on my cannon barrel just to remember which bolt to pick.
I would guess in my case the upper is out of spec, perhaps due to a coating issue? OP may have the same issue.