The problem with the 300, in my opinion is the light for caliber bullets, and questionable expansion of heavier bullets. A 125 grain Sierra expands well at 100 yards, but tends to come un hinged at close range. Heavier bullets hold up better at close range ( federal 150 grain factory ) but expansion is questionable.

Your in between a rock and a hard place with caliber restrictions. My vote is for something like a 375 JDJ if you reloaded but you wouldn't be legal.

I don't know enough about a 460 to say one way or another, other than it's probably not a good choice if you are new to hand cannons. I've shot them and The concussion is unreal. Recoil isn't bad but it's hard to concentrate when you know your going to get your ear drums blasted out even when wearing plugs and muffs.

Sorry, I didn't see that you already own a 460 Smith. An encore in 454 or 460 may be the way to go.