Random thoughts;
USGI mags are aluminum, not steel.
Full mags are sometimes hard to seat fully. You have to smack them just a bit when inserting. I load twenty round mags to eighteen, thirty round mags to twenty eight. Doing this allows them to seat with less effort.
Any shrinkage from cold would allow the mag to seat easier. Course, the rifle might be shrinking too...
Best AR mags- varies a bit from opinion to opinion. My choices;
LaBelle Teflon thirty round mags. The best, hard to find. Also come with Bushmaster or Defense Procurement Manufacturing Services floorplates.
DPMS Teflon mags. Different floorplate, not as valued because parentage is disputed. Buyers like them.
Colt, Bushmaster, LaBelle, Adventure Line, OKay, Center Industries, FN, or Parsons brand thirties. USGI powdercoat mags.
Colt, or Adventure Line twenties, powder coat.
I don't like the nylon ones. Others do. JMHO
Coopers have certain batches that give problems. Fewer welds on the spine caused some mags to crack. Some guys like 'em, most don't buy them.
There are new thirty rounders on the market that are steel, made in England. They are pretty cheap and seem to be good mags. Made for their armed forces. Not to be confused with steel aftermarket mags made in this country.
There are also Sterling thirty and fourty round steel mags. They are made in England as well. They are expensive and very hard to find. Very high quality, very collectable.
