Of all the deer I've killed with buckshot, I've seldom seen a pass through or really anything like 12" of penetration, yet the deer goes down like it was hit with a Mack Truck, and upon closer inspection is dead, dead, dead.
How many deer have you shot from an unusual angle with buckshot, like quartering away, facing directly away from you, or standing over you with a weapon while you're knocked down on the ground? How many of those deer had their arms raised at chest level, brandishing a weapon at you, so that you had to shoot through an arm to hit vitals?
For hunting game animals when you can pick and choose your shots, enough penetration to go from one side to the other is fine.
For protection against dangerous animals, humans included, you want a load that can punch through enough muscle and bone to reach the vitals from any conceivable shooting direction (you may be shooting to save someone else's life, remember), and also through any limbs which might be in the way.
12" to 18" is the established acceptable penetration range for the FBI (for shooting humans), with 13" to 15" being the optimum.
Just as reference here's the calculated penetration depths in 10% gelatin, for the common shot sizes at 1300 fps, assuming minimal deformation, using MacPherson's equations.
#8 -- 3.3"
#7-1/2 -- 3.5"
#6 -- 4.2"
#5 -- 4.8"
#4 -- 5.2"
#2 -- 6.0"
#BB -- 7.8"
#4 -- 10.9"
#3 -- 11.5"
#1 -- 14.0"
#00 -- 15.9"
#000 -- 17.1"
Keep in mind, penetration in real tissue may be shallower than in gelatin, especially if you're using unbuffered, unplated shot. The 12" standard is only for gelatin. As mentioned, there was a test involving meat, where 7-1/2 birdshot failed to go through 1.5" to 2" of meat. Even a 2-3/4 dram #7-1/2 target load at 1145 fps would penetrate 3.2" in gelatin, so for birdshot at least, penetration in pork and bone is about half that. A study done on 9mm 147 gr Winchester JHPs found that penetration in living humans was almost identical to in gelatin, though.
From Mossberg 2008 Gun Annual, page 24:
Some may offer that a load of birdshot with hundreds of pellets is a good load for "close quarters shooting." One ounce of #8 lead shot contains roughly 410 pellets, the equivalent load of #7 1/2 shot has slightly less. Yes, a load of birdshot is very loud and a contact shot would be devastating with the burning propellant gas doing a good amount of damage. However, a loud noise doesn't stop deadly felons and a contact shot is not something we can count on.
To test and see just how much penetration I could get from a birdshot load I went to my local grocery and purchased some meat. Fortunately, the butcher had packages of pork neck with bones and meat for a good price. (I couldn't bring myself to shoot up a nice piece of steak).
The meat target I created was secured in place with cellophane wrap on a cardboard backing. The density of the meat varied from 1.5 to 2 inches with bone mixed in. From a distance of 5 feet I fired a single round of #7 1/2 birdshot. The force split the cardboard and knocked the target over. Upon closer inspection I discovered that the meat had completely absorbed the majority of the shot. A few tiny pellets punched through the cardboard around the meat but none passed through.
What does this translate to?
At near contact range, a mere 5 feet, the #7 1/2 lead shot did not have the power to penetrate two inches of flesh. There is no way the shot would have reached deep inside an animal or human to shut down the heart, lungs, or central nervous system. Surely the wound would be horrible, but self-defense is not about inflicting horrible wounds, it is about stopping the bad guy from killing us.
For stopping a bad guy, looks like #1 is the best. It's right in the middle of the optimum 13"-15" in gelatin figure. If you'd rather err on the side of caution, there's nothing wrong with 00 and 000.
But for minimizing danger to other people if you miss, all you can really do is use a non-firearm, like a spear or a crossbow.
I actually have an idea for a shotgun load which would penetrate only 1-2 interior walls, while penetrating 12"-18" in gelatin (and criminals), but I'd need a lot of funding and crap to actually make it a reality.
Also,
The next time your reloading, load some 38 special hollow base wad cutters, in the case backwards. They are under powered enough that they won't go into the next apartment. If someone forces you to use them, a hit will leave a pair of ears on the floor and not much else!
Yeah, no. The big old hollowpoint looks intimidating, but they underpenetrate
if they expand. They get clogged up with heavy clothing pretty easily. Remington 158 gr LHPs are superior in every way, including expanded diameter.
Four .38 SPLs stacked 2 x 2 in a 12 gauge shell, right side up, with a light powder charge, could be very interesting, though.
