Head shot on bison
Ken,
If I may add some personal experience?
I did a lot of head shots on blesbok on the open plains for the venison market. Assuming that your shot shall be from about 200 yards in open grassland, fenced, I shall go for the brain. Sight your rifle at 200 yds, knowing it shall be about 1.50 inches high at 100. Wait for a side on shot and take it calmly on the line between the ear and eyes. If in doubt go for the earhole. A brain shot is indicated by the hind legs folding in immediately.
If you happen to miss the brain the shock shall put it down, but only temporary. This shall be indicated by the front legs folding first, or all four folding simultaneously, but not as sudden as the rear legs in a brain shot. Get close quick and shoot it into the brain then.
I do not do brain shots on game in the dense bush where off-hand shooting is called for, as a possible shot through the sinuses will not put the beast down and make it run and it may be impossible to find.
Of course the heart-lung area is a vital zone, but no animal shall drop immediately as the haemmorhage has to take effect. If it was a good shot and the animal bursts off at speed, just let it go, as following up causes axtra adrenalin that will boost it along. I have learned to NOT do any immediate follow up shots. Sit back, have a quick coffee from the flask and remember where the crosshairs were at the moment of the shot. By the time you are done the beast shall be dead about sixty yards from where it had stood.
Oh, and I do not approach any fallen animal head on after any shot, always slowly from behind.
I bet the bison is good meat, and a head shot shall ensure that it is all the better. Consider using a rest like crossed sticks too, for that head shot.