Nate,
Patterns will be pretty wide at 100 yards out of any shotgun I have ever seen, and if that weren't enough of a problem I expect penetration would have fallen off a good bit too- especially with smaller buckshot pellets. I moved up to 000 buck when I was experimenting at 100 yards, using 3" magnum Federal Premium loads (10 hardened plated pellets per load) out of an 18" barrel with the forcing cone extended and a Patternmaster choke tube. That was getting me 40-50% of pellets into E type silhouette targets at that range, with pellets sometimes penetrating the 2X4" and 2X6" boards the target frames were made of. I never went any further than 100 yards out with buckshot, and mostly stuck to the 000 buck loads. I think you're getting into slug territory out that far, and you are definitely into 'area target' versus 'point target' mode with all the shotguns I have ever seen. I would not consider hunting with buckshot at that sort of range, though it might be OK for antipersonnel applications.
I would like to do some work with Remington's HeviShot 00 loads, according to the press they offer significantly better penetration than lead pellet loads. But at a real premium price... and I thought the 3" Federals were premium.
lpl/nc