Dtom Oh BS on that phone guy...you think DPMS and its gen2 stuff could get away with that to the general public?
Ive got a couple of hybrid ar308's that yes are damn finicky on mags,but the lancers work perfect in them BUT you didnt buy or assemble a weird rifle...you bought an off the shelf rifle that should work pretty damn good...although these 308's are finicky on break in...the nothing less than 168 grain is just a plain stoopid azz statement .....so okay lets us figure out what to do...you seem to have a handle on the problem...so you said no lockback with one round in mag ?
gotta start there... can be a couple of things causing that....I would get a different mag like a metal one just to check...you dont wanna spend nearly 50 bills on lancers......it can be a gas problem or a mag problem...I would think those nitwits would put the right spring and buffer in it from the factory...are you the first owner? and I would get some heavier grain bullets just to comply and also get that out of the equation....if it works with heavy grain than you can eliminate the mag...one change at a time though... Wash