A friend of mine had a load that used two .455 (or so) round balls in a 28 Ga.....the standard load for the 28 Ga is 3/4 Oz but loading data goes up to 1 Oz........Sorry, I can't remember what the load was but with a little head scratching one might be able to figure it out.....he actually shot two round balls from his 28 Ga.....It was an O/U and he said he had four chances to hit his deer with two pulls of the trigger.....LOL

Personally the advice to go with the 20 Ga is good.