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When I shot it the few times I did, there were no marks that I saw on the casings.
Now there's marks quite all over the casings. Even took a chunk out of the bullet at one point. Like I said, it's a mess. I think it'll be going to a smith for a second opinion.
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the barrel-extension feed-ramp overhang is not ideal, and could explain why a chunk was taken out of a bullet. but that would not have anything to do with why you're having to mortar. unless the slight misalignment of the feed-ramps indicate that the barrel extension not being clocked 100.0% is keeping the bolt from being unlocked, but that would have to be a very finicky BCG (on the edge of specs for clocking angles) for that to happen, I've seen more feed-ramp misalignment and not affecting bolt unlocking. could be tolerance stacking.
but that feed-ramp overhang is definitely bad news, but unrelated to needing to mortar.
I've built over ten 7.62x39 AR-15 uppers, and they all work fine, various configs, and not had the problems you're observing, especially since you said the headspace checks out good, verified with HS gauges.
on all my 7.62x39 builds, i do deburr and take off the sharp edges of the feed ramps IAW
Hellbender's guide, make sure there's no feed ramp overhang, check headspace with gauges. I've got a couple of builds with Faxon barrels, and for most, if not all of my builds, I've been using Aim Surplus BCGs.
I also hit the mag feed lips with a stone file to knock off the as-stamped sharp edge which tends to carve out gouges on the case, who cares about that since you're not reloading the steel-case ammo, but sometimes it prevents the round from tilting up the feed ramp, not being released (bolt over-ride). Again, not related to mortaring.
if your headspace checks out ok, and the bolt is not unlocking, might be something with the cam slot of the BCG being slightly off, or the radial angles (clocking) of the bolt lugs being slightly off, so that when the bolt carrier comes all the way back. and tries to pull the bolt out of the barrel extension, the lugs are still just slightly engaged, overlapping, preventing unlocking. If so, then I suspect the mortaring is not to unstuck a stuck cartridge, but rather to jar the bolt head to get it to unlock, to jiggle the bolt to get the lugs to dis-engage. You might see evidence of that on the back-side of the bolt lugs.
sorry, that's all that comes to mind.