I'm the op. I've experienced this problem with only two shells in the mag tube while duck or dove hunting with heavy loads. Shells surging or moving toward the front of the tube becomes a problem if and only if the action moves rearward that specific short distance and activates the first shell latch. This latch then catches the shell that would have been released upon "pumping". Maintaining forward pressure on the pump handle or fore end eliminates the problem. I accomplish this task by extending my left trigger finger along the side of the fore end. This position provides the momentary hesitation needed to delay opening the action.

See 26thInf's post in this section for the method he taught to prevent the occurrence. All shotguns with a mag tube have shell surge upon firing heavy loads. To my knowledge, only the 870's design causes a problem. I am an apologist for the 870 because it's my favorite shotgun. I can teach anybody how to drive one. 26's method is better for cops or soldiers.