Excellent gun! Look on the side of the chamber. It "should" say 2 3/4 and 3" shells. Since you mentioned Magnum, then I assume it's safe for 3" shells (which technically are the magnum variety anyway).

I have the Remington 870 Express Magnum 12GA. It takes 2 3/4 and 3" shells without a problem. Incredible shotgun - it goes anywhere, does anything, shoots anything, without ever having a problem!

Butch