I have made three 4 bore rifles and four 4 gauge shotguns. With a service load, the recoil is a bit brisk. In fact, it's so brisk I won't zero in the rifles any more. I leave that to the customers. Here's a picture of a man shooting one I made. It's loaded with only 350 grains of powder in this photo. The service load is 425 grains. A “magnum load” is 600 grains, but no one was willing to try that,
least of all me.This man is 5' 9" tall and weighs 175 pounds.
You'd done have to be a rocket scientist to figure this physics problem out.