You'll find most any detachable magazine system designed for military use will have some slop or play in it as they're designed that way. Soldiers don't want to have to fight to get a magazine in or out of the rifle during the heat of battle so that slop allows a little wiggle room so empty mags will drop free easily and fresh mag's can be inserted a little easier w/o binding up.

AR mag's rattle a bit
M1 mag's rattle a bit
AK mags rattle a bit

Nature of the beasts...