Do an experiment: Buy a watermelon and shoot it at 20 feet with a 12ga load of #4 bird shot. it is quite impressive and far more powerful than what any SD handgun is capable of ( .454, .475, .460, .500 excluded )
Great, next time fruit riots in the street, he'll know he's prepared
This is a pic of #4 birdshot shot into callibrated ballistic geletin at 3 meters with an 18" barreled Improved Cylinder bore shotgun:
Not impressive. The FBI asks for 12 inches of penetration to reliably reach vitals from any angle. Some other agencies ask for only 9 inches. Just about everyone kinda hopes for more than this. Massive, shallow, ugly, and painful--yes. But not deep enough to be depended on to reach vitals and actually put a guy down. Consider that on adrenaline, or some drugs, pain reception is altered and delayed. Barring CNS hits, just about anyone who relies on a firearm for a living knows you have to destroy vital organs and vessels to put a man down. The testers had this to say about birdshot for "tactical" or defensive operations:
Small sized birdshot such as this #4 heavy dove load is a poor choice for deployment with a tactical shotgun. Wounds inflicted from birdshot tend to be gruesome yet shallow as they lack the penetration required to reach vital cardiovascular or central nervous system structures.
A tighter choke might help, but while the shot hits in one solid clump, each individual pellet is round and very lightweight, which means it loses momentum very fast. Ballistic geletin is scientifically proven to simulate flesh and more often than not, street results back its results. Humans aren't homogenus, but striking bones isn't going to help penetration any.
I've shot a whole lot of birdshot. A whole lot. I've shot hundreds of ground squirrels, if not thousands, with my trusty Rem 870 Wingmaster loaded with 7 1/2 shot. This is smaller than #4, but not by a too much. My shotgun has an extra full turkey choke. At 25 yards, the vast majority of the shot lands in a pattern I can put my hand over. But I've seen plenty of ground squirrels flop over and squirm down their holes taking centered patterns at that range. And having shot fail to penetrate a 1 or 2 pound varmint is the rule, rather than the exception. I've seen birdshot fail to stop varmints enough times at distances I have in my house, say 30 feet from my bed to the living room, to trust it against anything bigger than a rabbit...er...ah...watermellon
Of course, I guess it is better than nothing.