Every shotgun performs differently, so you need to pattern your gun with whatever loads you want to use, at every distance you deem realistic to your situation. The go-to defensive shotgun load is 12-gauge, 2¾-inch 00 (double-ought) buckshot, which sends eight or nine .33-caliber lead pellets, each weighing roughly 53 grains, downrange in an ever-expanding cloud, somewhere between 1,150 and 1,600 fps.
Traditionally, with old-school full-power buckshot, you could expect one inch of pattern per yard from the target—a ten-inch pattern at ten yards, twenty-inch pattern at twenty yards, etc. Tighter chokes were a simple way to improve this, but police shotguns generally have cylinder bore or improved cylinder choked barrels, and fixed chokes. Federal decided to fix the problem by improving ammunition in the 1980s, when they began offering…
