I've only been shot a couple of times...
One time I was hit in the side of the head with a single #6 pellet during a pheasant hunt. I don't know the range it was fired from, there was a lot of guy's doing a lot of shooting and it probably bounced off of a tree, before hitting me or otherwise was fired from a long distance, since there where no shooters on the side I was hit from. So I'm guessing it somehow bounced off the tree.
The pellet only broke skin and didn't penetrate bone, at least not very much. One of my buddies dug it out with my Swiss Army knife, but, I'll tell you this, it "felt" like I had been hit upside the head with a high speed baseball, bad enough to knock my off the pheasant I was shooing at... got him on the next shot though.
So...there was a definite whack to the side of the head, I can only imagine what a full on impact would have been like...or something larger then a #6 pellet, but I have no doubt it would have the potential to knock a person down...or at least rattle one enough to cause one to fall.
Not scientific I know, and before anyone else says it...now you know what's wrong with Jitterbug.
The other time I was shot was once again with an errant pellet fired from at least a 100 yards away on a dove hunt, and it didn't penetrate the seat of my hunting pants.