Shotgun effectiveness is totally depended on the type of shells used. I would NOT want to get shot with a 12 gauge and 00 buck.
Especially 3" magnum. And distance from shooter is critical. What
Real Dave described, is a relatively common hunting accident in which he could have been hundreds of yards from the shooter.
What he was hit with ("BBs") probably was #5
birdshot. If the impact was hard enough to knock him down but there was so little penetration the clothing he was wearing would need to be comparable to body armor.