A 12 gauge is 18.5 mm. a standard rifle round ranges from 5.56 to 7.62. I’m not sure about them times but I’m pretty sure a repeater was a .308 so their rounds would of been 7.62
That’s over double the size, I’m guessing you already know about the spread, as that is what makes shotguns close range because of the pellet spread. But if you were to hold that point blank it would make a bloody mess of someone’s head. Might not quite disintegrate it but it would certainly be a mass of strung out skin and shattered skull everywhere. If you were to change the round to a slug, that would be a single size shot, so would leave even more of a mess.
