What Einstein even thought that a 12ga would work? Seriously? ? Buffalo hunters of old used high powered rifles - Sharps, Springfields and the like - for a reason, they work, lesser rifles and shotties do not.
Buffalo/bison are dangerous, and not just to cars. A neighbor's cousin had 3, put them in his Dad's corral. He had to leave for a few days but he told his Dad - if they get out, just let them roam, the son would gather them when he got back.
Sure enough, they got out, but Dad was an old farmer familiar with livestock, so he and a neighbor rounded them up and drove them back to the corral. Dad opened the gate and stood there waiting for the buffalo to pass to then close the gate. As one of the buffalo charged into the pen, the critter tucked his horn and gutted Dad on the spot as he ran past, died right there in moments. Cows typically don't do that unless you're in a bullring.