You would want to have a good foot hold, your dentures well glued in and have a death wish to shoot that off your shoulder
I was talking with a mate who fired a 4 ball once off his shoulder
it threw im on his butt.... his mate fired it (photo on his wall to prove it)
The guy broke his nose & thumb, the photo shows him on the back foot, front foot 1' in the air (about to fall over) muzzle pointed straight up in the air (not on purpose) still with the muzzle flash visible!
Evidently, guns like 4 balls (yes, 4 balls from 1 pound of lead!) were usually mounted on the railing of boats or trucks (for obvious reasons)
I have a .662 smoothbore (16.8mm round ball/16ga) muzzle loader, so I can imagine what an 8ga kicks like