As for the caliber, go with any of these: 44mag, 454, 480. Your shot my have to quick, so stay away from the 500 from S&W. The X-frame is way too big and bulky for a quick shot. And I'd also go with a double-action, you may not have time to cock the gun. There are several different choices that will fit the bill. Ruger, S&W and Taurus all make fine DA revolvers. If I were to choose one, I'd go with the Ruger SRH in 454 with the 7 1/2 barrel. But, it really comes down to what "fits" you the best.