I have a remington 870 express 12 guage 28" barrel pump shotgun and was wondering if it will shoot 3" shells. On the barrel it says it will shoot 2 3/4 or 3 inch shells in magnum receivers and 2 3/4 shells for all others. I don't know if the receiver is a magnum or standard. The model is just a 870 express. How can i tell? thank you.