A bit late to the conversation, but I'll throw in my 2 cents.

A 300 grain bullet that's within its expansion window is going to kill any whitetail in the country, even at the lower end of its velocity range. For the Swift A-Frame -advertised to still expand somewhat well down to 950 fps- and using the Federal factory load advertised at 1,750 fps -which was likely chronographed out of the 8 3/8" barreled Model 460 revolver- that gives us 950 fps out to 250 yards. MatchGradeMachine's velocity testing with a 300 grain handload out of a 15" barrel yielded 2,164 fps, which would retain 950 fps at about 325 yards.

The limiting factor in my opinion isn't power, or even accuracy. It's the bullet trajectory. At 250-300 yards, even out of a 460, the bullets are experiencing rapid bullet drop, to the extent that a 10 yard ranging error can mean a hit 3-5 inches high or low. That's not even getting into managing the wind, which will definitely come into play at those distances. Like Ernie said, as long as you've practiced and can keep your shots within the kill zone at the ranges you intend to encounter game, you'll be good to go.