that sounds about right, mags of more than 30 rounds seem to jam more often.... thats why the military mags are 30 round max. from the m-14 to the 16....(tested for limitations).. also where the tall tale of loading 1 or 2 less rounds comes from. the saying that the springs will decompress and cause an ftf. whether true or not, it has its place of origin. also why belt fed is more often used (besides the fact you can reload and maintain a constant stream of fire with belt fed set-ups....... until the barrel melts anyways...lol)..... but regardless of truth or myth, drums are heavier and more prone to jamming than lower capacity mags or belt fed boxes of ammo.. of course with newer tech, design and metals used it may be different now