I have to back up DM on this one. Some people fail to realize it, but there is a whole lot of energy that comes out of the business end of a shotgun. The only reason why its difficult to kill a turkey at 40 yards is because the pattern density is decreasing and you are aiming at a very small target where at 20 yards you might get 15-20 pellet hits versus 1 or 2 at 40 if the shotgun doesn't have an extra tight choke.
At even closer ranges, the shot column acts almost like a single projectile when it comes to penetration.

I shot some large pumpkins (uncarved) with birdshot, turkey loads, and a 30-06 last fall. The shot shells did much more visible damage to the pumpkin at 20 yards than than the '06 did at 100 yards.