Ah, I think I'm in love.
Shot out to 500m today, making first or second shot hits on steel dialing in the come-ups as I went from 300m to 500m. My buddy, who never fired an M14before today, had no issues hitting at 500m. And that was with the cheap Norinco steel-cased ball! All is good in my M14 world, for now.
I've got a new gas cylinder and piston coming for my old rifle. I think that may be the root cause of my failures-to-feed.