I know you guys flunked math, but let me TRY to explain to you what that 20" pattern MEANS. The Area of a circle is determined by multiplying "pi" (3.14) x the square of the radius. The radius being half of the diameter. So a 20" circle has a 10" radius. That means that the 20" circle's area is 100 (10 squared) x 3.14, which comes out at 314 square inches. Your 9 pellets of 00 buck are randomly distributed over that 314 square inches, so there's 35 square inches for each pellet. The chest vitals of a man's chest are either 12" x12" if he's frontal, or about 8"x12" if he's sideways. For convenience, we say it's a 10" circle. No, dummies, a 10" circle does NOT half half the area of a 20" circle. It has 5 squared x 3.14, or about 78 square inches. 78 isn't even 1/4 of 314 dummies.
So what you get is at best TWO lousy 00 pellets on the chest being all you can expect. At 20 yds, the pellets, even if they WERE started at "magnum" velocities of 1400 fps, are down to 1100 fps at best. Each 00 pellet weighs a whopping 60 grs, so each pellet, at 1100 fps, has a whole 160 ft lbs. From a rifle barrel, a .22 stinger has 160 ft lbs, guys. So would you EXPECT a couple of .22 stingers to the chest to stop a serious attacker, hmm? hell no, and you likewise shouldn't expect a couple of 00 pellets to do so, either, for the same reason. They just aint GOT enough poop to incapacitate a man at that range.
You can't COUNT on buckshot stopping a man, unless you get over HALF of the pellets into his chest, and bud, given errors in aiming, POOR sights and triggers on shotguns, lack of adequate practice, and typical shotgunner's laziness and bs about pointing it north and hitting south, that 50% hits on the chest does NOT occur at much range at all. NOt even at ranges that are EASY head shots with a good rifle. Like at 50 ft.