There's an excellent post over in the turkey hunting forum on how to pattern a shotgun, it's a sticky so it's at the top. Use something like posterboard, 24x36, and use the loads that you're going to hunt with. Since you're worried about point of impact shoot from a distance where your pattern puts all of the pellets in a 12" circle. The barrel length has nothing to do with the pattern, the choke determines the pattern. The 870 might just fit you better so it hits where your pointing.