FWIW, this is from Remington's website:
If your shotgun's serial number ends in M, N or U, then you have a 3-inch magnum shotgun and will need to use magnum ammunition to make it cycle correctly. (Be sure to double check the barrel for the proper size shells)
The magnum barrel only has one gas orifice hole in the gas chamber and a field load will not supply sufficient gas pressure to cycle the action in a magnum barrel. Field barrels, on the other hand, have two orifice holes and are designed for the the lighter 2 3/4-inch loads.
