been playing around with the mags on other lowers--so far the work fine in everything else.
thought it might have been some kind of out of spec upper issue causing the BHO issue.
the upper is a very old colt slab-side i got for $20. it does have an out of speck front pin hole and a slightly angled fore and aft pin location.
the thing is the upper works well with the BHO on other lowers.
i got this TNW billet lower from CFS years ago for peanuts , never noticed any problems with it . in fact thought of it as one of my best lowers.
now it turns up "slightly " out of spec--wth?----LOL
i'll just keep the plastic mags out of it except for the thermelts--LOL!