When I was a kid, I got kicked by a horse.
Looking back at that, I believe it was a good natured thump designed to train me. I'm quite sure that horse could have killed me if he wanted to.
As far as comparing that half throttle thump to the recoil of a rifle...
I've fired the 300 mags, the 7mm mags, the 375H&H mag, .45-70, maximum loads in my .50 cal muzzle loader, magnum shotgun loads, .44 mags, .50 AE, .454 Cassul, and even fired .45 Long Colt out of a derringer.
None of those bruised me like the horse did. Not even close.
I'm going to have to agree that "Kicks like a Mule" is an exaggeration.