A humorous blog post from Dave Petzal:

If you are not familiar with the 8-bore, it shoots a 1,250-grain conical bullet at about 1,500 fps, or an 860-grain round ball at 1,650 fps ….. This year, one of the .375 shooters got the chance to try it, and asked me if he should.

“Sure,” I said, “it will take you into a whole new world of pain that you never dreamed of.”

The round ball shot out of an 8 bore has a diameter on .820 inches! That is just over 20mm!

In one of the comments ThreePutt wrote:

“You teach a good lesson: One mans kick is another mans recoil.”

Very well said, I could not put it better myself!

Some of the other comments pointed out the dangers of both perceived recoil (how bad it feels to you) and actual recoil. I recommend reading the comments.

Below if a picture of an 8-bore rifle, it may be a Greener, but I am not sure: