Oelaf,
I've used the 300 grain swift/9.3 on many Alaskan game. I've never used them on buffalo though. They expand well down to 1500-1600 fps. They've penetrated deep. I'm not sure if you can get ahold of alliant 2000 Mr, but that's what I've used. 64 grains for 2400 fps.
Reloaded 15 or reloader 17 works fine too:
https://www.24hourcampfire.com/ubbthreads/ubbthreads.php/topics/13539716/1
