i had the same problem with my 870 express super mag. when i first got it, it would randomly lock the pump up with 2 3/4 shells. i had no idea wut the problem was. but i jus kept shooting, and if it jammed, i would rack the pump realy hard and it would let loose. after a while i never had the problem again(no matter wut ammo i used). seemed like i jus had to break the gun in, cuz now i never have ne problems with it.