I have noticed the same. I shot a person with a mastiff at point blank (weapon was touching the enemy player) and only did 60 damage. With peacekeeper, I sometimes get multiple damage numbers, other times just 10 damage at close range and when aimed at the middle of the body of the enemy. It seems to only be with shotguns though, so there's something wrong with them.
Now I see some people saying this is because it's a close quarters weapon, yet I see multiple damage numbers when I shoot at what I no longer consider close quarters. Suffice to say, I avoid using shotguns.
