12 gauge only comes in 2 3/4 or 3"
if you want to step up to 3 1/2" shells your gonna need to leave popcan police pump action junk at home and get yourself 10 gauge.
Remington might tell you there is no extra benefit in a magnum round but a 3" shell will cut a wood post in half at 25 yards and 2 3/4" shell just peppers it.
honestly that price is pretty high for what is most likely a heavily used shotgun you can pick up an 1100 for a little more and it does all the work for you.