I sometimes use an old Webley senior .22 air pistol for humane dispatch in cage traps, but the range is only a couple of feet maximum. It runs at about 4 - 5 ft lbs and I won’t use it on squirrels because I don’t believe it has enough punch to put them down instantly. It seems to do rats ok, but as others have said, use a rifle. I wouldn’t attempt to shoot rats, even at a few yards with an air pistol. Even if you’re a crack shot with a pistol, the striking energy of the pellet would be marginal at best in my opinion.