Test different sizes to determine what's best for your purposes.
Just got to thinking tonight about the gool ole' 12ga. Using a standard slug (in a smooth bore shotgun), how effective would it be at say 75 to. I personally cannot advise a shotgun as a close range defense. Many people cannot handle the kick, noise or the thought of shooting someone. Shooting skeet or range is a vast difference.
Shoot one of your shot shells at a piece of paper big enough to record the impact of all, or nearly all, of the pellets at the range you expect to use the load. At 40 yards (36.6 m), expect to get a pattern with a diameter of roughly 20-40', depending on the size of the shotgun shell used.- Examine the pattern. How big is the main cluster of holes? Will it be hard to hit a moving target with the pattern? How dense is the pattern in this main cluster? Will enough pellets hit the target to kill/break it? Is the pattern uniform? Are there large holes in the pattern where targets could escape?
- For slugs, shoot between three and five shot groups at paper at a reasonable range; say 50 yards (45.7 m) for rifled slugs, 100 yards (91.4 m) for sabot slugs. Both types of slugs are fairly low velocity, and thus have rainbow-shaped trajectories that are not helpful to long-range shooting. Pick a slug which produces acceptable patterns and satisfies your desire for target damage.