I had an 12" Encore in 480. Buffalo will be no match. with the factory 325g jacketed take only broadside, 400g quatering is no problem. Now with 400g-440g Hardcast Bullets around 900-1200fps {Light Loads} you will easilly surpass the factory jacketed rounds with considerably less fuss. If you can hit @200yds this round will take them, learning trajectory, and practicing at that distance will allow that. Though handgun hunting is a close range encounter for myself, 50yds is far for me 75yds and I need a rest/bench.
A fine choice though, if you hunt'em close 420g WFN @ 900 fps will be great.