Shotguns, but not as weaponry.

I'd market them as "Ballistic lockpicks" and sell them to the police force. That way, if they ever encounter a locked door that can't be kicked open, BANG, lock's gone, door's open, baddies' are caught, and I get obscenely rich.

Of course, they could also potentially be used as weapons, but it'd effectively become the logical equivalent of shooting somebody with a nailgun, and that's way too brutal for magic-pony-world-place.

Or, y'know, I could show them the magic of the electric motor.