Just for fun I weighed a #4 pellet of birdshot, 3.1 grains on my scale, then I weighed a #2 pellet, an even 5 grains is what I got. So 546 grains of lead give or take, or about 110 individual pellets capable of penetrating 4-5" at 50 yards. Not all will hit the target, but I found at least 14 pellets in the coyote I had shot, 2 had penetrated the skull, at 50 yards. At 20 yards even with a mod choke at least half the pellets would hit inside a 16" circle with a couple more inches of penetration without velocity loss at 50 yards. Would #4 Buck be a better load for defense, absolutely, but #2 birdshot is nothing to sneeze at inside house distance, at 20 feet it would leave a 4" gaping hole about 6-8" deep even through clothing. Now OO buck might very likely go entirely though a human at that same distance, good or bad you decide.
Just a comparison, I am a big fan of the .17HMR. I use the 17 grain bullet starting out at about 2400fps, at 250 yards it is down to about 1150 give or take. I have found that past about 190-200 yards the bullet has slowed to where when it hits an animal it doesn't expand or break up, it just pushes through. I have killed 3 coyotes with this little gun, the furthest was at 260 yards where the bullet just poked a .17 hole in one side and out the other in the chest cavity. It dropped and laid still for about 5 seconds and then got up and ran 50 yards and keeled over dead. Now that single 17 grain bullet is traveling about the same velocity at 250 yards as where the same load of 546 grains of #2 shot starts out at the muzzle of the shotgun.