Buckshot is similar to but larger than birdshot, and was originally designed for hunting larger game, such as deer (hence the name). While the advent of new, more accurate slug technologies is making buckshot less attractive for hunting, it is still the most common choice for police, military, and
home defense uses. Like birdshot, buckshot is described by pellet size, with larger numbers indicating smaller shot. From the smallest to the largest, buckshot sizes are: #4, (called "number four"), #1, 0 ("one-aught"), 00 ("
double-aught"), 000 ("triple-aught") and 0000 ("four-aught"). A typical round for defensive use would be a 12 gauge 2 3/4" (7 cm) length 00 buck shell, which contains 9 pellets roughly 8.4 mm (.33 inch) in diameter, each comparable to a
.38 Special bullet in damage potential. New "tactical" buckshot rounds, designed specifically for defensive use, use slightly fewer shot at lower velocity to reduce recoil and increase controllability of the shotgun. There are some shotgun rounds designed specifically for police use that shoot effectively from 50 yards (46 m) with a 20" diameter grouping of the balls.
For SD i wouldn't use anything smaller than the #4 shot. I have #4 and 00 here at home for my defense puropses.