The hardest kicking rifle I have shot (I don't own them, I made them for others) is the 4 bore.
One shot convinced me completely that I don't want to own one.
Here's a pic of a friend firing one with a light charge (not kidding either, this was a 60% charge)
He's 5 foot 9 inches tall and weighs about 170 pounds.

I am starting on BigDog's 2 bore. There's NO WAY I am going to fire it. I will test fire the barrel, (tied to a tractor tire), but I am NOT putting this thing against my shoulder!