Okay, posted previously about my 605 Clone (16" retro carbine with rifle length gas system). Bought it at a gunshow (I've eliminated the vendor and original owners as the sinners here; it was never fired). It came sans BCG, so I put a new LMT one in.
Gun wouldn't run; fired round was ejected, but the next one wasn't picked up.
I had a lightly used, vintage Colt BCG and tried that; worked fine.
Put some new gas rings in the LMT carrier group, and now it runs fine as well.
Ideas?
Moon