I've done a few penetration tests over the years and once shot a 12pellet 12ga 00 buckshot into a clothes closet. Most made it trough the drywall, the ones that hit studs were stopped, a few made it about 6-8" into a down jacket hung sideways. The old leather riding jacket stopped them about as well. Not one made it through the closet.
Old car bodies are also and interesting target for penetration tests. Speed penetrates. With my 25/06 the 90gr bullet would shoot through a 1/2" steel plate at 50yds. But the 120gr load would not. Buckshot will not penetrate old car bodies but will some of the newer ones, however many times will still not make it into the car interior being stopped by insulation and interior pannels.
I'm a believer in slugs regardless of gauge. A friend was with a entry team at a drug dealers home a few years ago. As he entered the bedroom the perp sat up in bed with a handgun in his hand. A 12ga slug ended his evil ways rather dramaticaly. Yep if your shooting a shotgun for real don't fool around with shot use a slug.
Regards,