Don't know about M1As, but most AR shooters use a dedicated single-load 'magazine' for the slow-fire stages - either a regular 20rd mag with a special follower inserted, or a solid block of Delrin machined with a tray to position the round correctly.

I don't think Fred's single-load mag is going to work in an AICS pattern mag system. A while back the people who made the MAK Tube gun chassis offered one, but it seemed unavailable more often than not. Might be worth googling, though.

Flash hiders, such as on an AR-15 A2 or M1A are allowed. Compensators, brakes, or other things that act to divert muzzle blast back towards your fellow competitor and/or reduce recoil (which is why suppressors get nixed also) are not legal on any NRA rifle.