The Big Horn Armory
AR500 rifle and the 10" barrel pistol version thereof look pretty interesting, especially if a character makes his own bullets and needs big ones so he can make hollow-points with enough volume in the cavity to fit a dose of silver nitrate, wolfsbane or whatever chemical happens to be toxic to the critter
du jour. He can use the same bullets for the .500 Auto Mag and, say, a back-up handgun in .500 S&W Magnum.