I think the hardest I got kicked was by a Shiloh Sharps in about 1986. The rifle had a standard weight barrel and weighed 8 pounds. It was a 50-140 and was loaded with 140 grains of 2f black powder and a 750 grain bullet. My nose bled and the whole side of my face and ear went numb.
I'm sure there are harder kicking rifles out there, but that was the toughest one I ever shot.
