Keep in mind that, like any other government body, the FBI imposes arbitrary standards. I'm guessing that in this case the arbitrary standard is trying to account for barriers and/or heavy clothing. If they wanted to be really scientific they would use cadaver testing, but that probably wouldn't go over too well with the public, so they use gelatin.
MikeT is spot-on when he mentions 2 yards (6 ft). If I had a choice, I'd prefer a bit more distance between me and an aggressor. Birdshot loses velocity quickly because the small pellets have less mass. At 20 ft (the length of a hallway), it may cause an aggressor a lot of pain, but might not be enough to end the fight.
MikeT is spot-on when he mentions 2 yards (6 ft). If I had a choice, I'd prefer a bit more distance between me and an aggressor. Birdshot loses velocity quickly because the small pellets have less mass. At 20 ft (the length of a hallway), it may cause an aggressor a lot of pain, but might not be enough to end the fight.