Birdshot

One of the attendees at the cookout brought his Remington 20 GA this weekend with just this question, we didn't have any true test medium, but we used it on cardboard at about 20 feet, with the short barrel we were driving the load through the card, still in the wad cup, now that's 7/8 ths of an ounce of lead pellets still compacted in the cup, I would imagine the wound would be similar to that of a Glazer Safety Slug, except bigger, and heavier, add in the shock and awe of the bigger "Boom" and muzzle flash of the shotgun over a pistol.

In a safe room the attack is very likely going to be a frontal attack, and penetration is not as big an issue, 6-8 inches is more than enough to hit vital organs on most people, and in most bedrooms the distance will be less than 20 feet, and I advised him if he had to shoot, to shoot until the threat ended, not to shoot & look to see what the results were.
Just to make him feel more at ease, I did locate some 00 buck loads for him on Sunday at the show.