I have never had any problem with any U.S. Surplus mags.They will have the locking lug? welded to the body.Stay away from aftermarket that stamp the locking lug into the mag body.
DON"T GO CHEAP I did to start and spent $80 on what I thought was a good deal, got 5 of them, and I had trouble with every one locking in place. The followers in two of them were incorrect and lined the rounds too far to the right, which caused them to feed incorrectly. I don't know what brand they were, but I got rid of them and now it is nothing but U.S. Surplus for me.
Checkmate Industries or CMI are the main mags that everybody seems to like most. They are american made and are solid mags and 44mag.com has the best price on them that I have seen. Also, it has been said on other boards that CMI is the actual maker of the Springfield Armory mags that you get with your rifle new.
Another vote for CMI from 44mag. They have been flawless for me. I've seen it mentioned many times that CMI is supplying to the military, also Springfield Armory.
44mag has some good prices on AR mags too