probably something like a pump action remington ultramag, but with settings that cause different bullets to appear in the chamber.
Setting 1) something beyond hollowpoint, like some kind of cup shaped bullet for maximum damage to soft targets at close range
2) A brass coated tungston round for maximum range and penetration
3) this cool bullet I saw on bones that has a copper/brass coating which peals away on impact to reveal a core of birdshot for maximum damage to soft targets at range
4) of course it would have a setting that fired shurikans and lightning, no weapon is complete without that.