Article: "Tips & Techniques for Defensive Shotgun Patterning"

Shooting Illustrated has an article for patterning of a defensive shotgun. As the author explains:

With a defensive shotgun, you need to know the maximum distance at which you might take a critical shot. Granted, a shotgun is not a precision weapon, but some loads in some shotguns will allow you to deliver exact placement of the entire payload. You also need to know the distance at which the pattern of a certain load exceeds the target zone. This is important for two reasons: First, we want to deliver the maximum effect on target. Second, we want to make sure there are no pellets straying outside the target, possibly impacting things or people you don’t want to harm.

He then describes using an IPSC target to determine your pattern at 10, 20, 25, and 30 yards, taking one shot at each range and recording and marking off the hits at each stage. By the end of the process:

What you end up with are three different zones of effectiveness. You’ll know the precision zone, which might allow you to take a head shot. You’ll also know the incapacitation-zone distance at which you can keep all of the pellets on target. Finally, you’ll know the distance at which you should switch to a slug because your pattern is too large.