I used 1/20 for all of my hunts. I want a complete pass through to vent the boiler room. they go down quicker.
The last one I shot I had to put three shots all three hit just above the heart on the lower part of the lung.
The Bison is a tough animal. If you are close enough where you feel confident to take a head shot behind the ear he will drop on the spot but if you want to mount the scull there wont be much solid enough to hold the hide.
I used the .44-77 for two and the .44-90 bn for the first. All three I used the original Sharps bullet profile patched.
IMG_2498 by
Kurt, on Flickr
IMG_1614 by
Kurt, on Flickr