I think it'd be pointless to include a 28-gauge revolver if it can't even one-hit kill/one-hit incapacitate. It's a 28-gauge. That's bigger than a shotgun round.
Shotgun rounds are commonly 12 or 20 gauge. A common misconception is that the 20 gauge round is bigger, but it's not. Gauges are calculated inversely to mm or caliber, in that the bigger the number, the smaller it gets.
It came from much older times, when you had to make your own ammunition. You took a 1 pound block of lead, and you'd shave that down to slugs of your shotgun's gauge.
12 gauge meant from a 1 pound block of lead, you could make 12 slugs.
20 gauge means from a 1 pound block of lead, you make 20 slugs, which would be understandably smaller, since the lead is cut into smaller pieces.
28 gauge would then be even smaller.