Magnum usually just means more powder and shot in the shell. There are 2 3/4" magnum shells too.
Shell LENGTH however is a different deal. You shouldn't fire 3" magnum shells in a 2 3/4" chamber- the hull mouth of the 3" shell will protrude into the forcing cone of the shorter chamber and can cause dangerous overpressure at the moment of firing.
With 12 gauge 870s, M is for 3" shell capable receivers. That is, if the last digit of your serial number is the letter M, then your receiver is coded for 3" shells. Remington has always used a code system to ID 870 receivers this way, by gauge and shell length.
If the last letter in your serial number is a V, then your 12 ga. 870 receiver is set up for 2 3/4" shells.
The only difference is in the location of the ejector and that's all, there's no strength issues with the receivers. Oft times V coded guns with 3" chambered barrels installed will load, chamber and fire 3" shells but then have trouble ejecting the empty. Remember, the shells aren't really 3" long till after they're fired and the crimp unfolds (and unfired 2 3/4" shells are usually only about 2 1/4" long).
To be sure of proper function with 3" shells, you need a 3" chamber in the barrel AND an M coded receiver.
hth,
lpl/nc
