Max, supermagnum is right. Any 870 barrel will fit your super magnum as long as it is the same gauge. I just picked a smoothbore barrel up at the last gun show that will take 2 3/4 or 3" shells. Most everyone tells me that I don't want to shoot 3" slugs anyway. I believe the reason is that they shake your brains loose in you skull........!!
Just don't go and buy a 2 3/4" chambered barrel and fire 3" or 3 1/2" shells out of it. You may end up with a very short barrel and some serious injury.