I just know the ballistics AFTER it leaves the barrel. At close range, the combination of hot exhaust gas and pellets acts like a single solid, cohesive object. As it travels and cools, it loses cohesion and starts to act like a swarm of individual round pellets - and "round" is a really sh------- ballistic shape, so the individual pellets lose energy rapidly.
Which is why Whittington still has a head. He was hit by a swarm of pellets. I don't know how far away from Cheney he was at the time...but another two or three yards farther, and he probably wouldn't have sufferred much more than having a handful of gravel thrown at him.