Not poo-pooing a shotgun for home SD at all, but they're are a some misconceptions with regards to pattern and penetration where SD is concerned.
A few years ago I read a post on the subject of home defense with shotguns loaded with birdshot. In the post, someone knew someone, who knew someone, who knew someone that was shot in the chest at very short range (6 feet if memory serves) with a load of #6 birdshot out of a 12 ga. The purported victim waited patiently for the ambulance, was transported to the emergency room where the shot was removed and he was then sent home, supposedly. In an attempt to show the poster that it would be near impossible to survive a shotgun blast to the chest at the range, even one loaded with birdshot, I performed a hasty, informal test by firing a 1 1/4 oz. load of #6 from an I/C choked 12 ga. at a piece of 1 1/8" sub-flooring at 10 and 20 feet-
Entrance-
Exit-
I had pretty good idea what the results would be because as a knuckleheaded kid that grew up in the country roaming the river behind our house with a shotgun, I'd shot at everything imagine able with shotguns and knew that at close range even birdshot turns things inside out. But what I learned from my impromptu test was a couple of things; First, a shotgun, even one with an open choke at 20', fires surprisingly tight patterns and would need to be aimed almost as deliberately as a rifle or handgun. Second, if a 1 1/4 oz. load of #6 will so easily penetrate a 1 1/8" sheet of laminated plywood, a couple of pieces of 1/2" sheetrock wouldn't even come close to stopping it, change that from #6 to buckshot and penetration would be even more dramatic.
Sorry about the digression, back to the subject of the .30 Carbine and its effectiveness or lack thereof.
We gun folks sometimes lose perspective on the subject of cartridges. We seem to forget that cartridges don't actually kill, the bullets launched from these cartridges do. One perfect example is the 10mm Auto. "Uncle" Ted Nugent used his 10mm to take a gemsbock, and wildebeest and of all things a Cape Buffalo, so now anytime the subject of a defensive handgun for anything smaller than a Tyrannosaurus Rex comes up, the 10mm is the most widely recommended cartridge. The heaviest load I could find for this cartridge is 200-220 gr. cast bullets at 1200 fps. I own a couple of 130+ year old 1873 Winchesters in 38-40. Either, loaded with a 192 gr. cast FN bullet and 2.2 grs. of Swiss 3Fg. a hair over 1400 fps. So tell a fellow gun folk you're going buffalo hunting with a 10mm or that you're carrying the same for bear protection while salmon fishing in Alaska, and you're liable to get a grin and two thumbs-ups. Tell them you're going to do the same with a 130 year old Winchester in 38 WCF and they'll probably volunteer their help in the search for your marbles.
And so it has been for years with the .30 Carbine. Loading manuals and magazine articles pretty much state that the cartridges usefulness is for plinking and maybe small game. Then along comes the .327 Federal Magnum (A .31 caliber revolver cartridge) and claims that revolvers so chambered will do marvelous things. Richard Mann seems especially fond of it-
"This means the .327 Federal Mag.’s projectiles will penetrate similar to 125- and 158-gr. hollow-point bullets from .357 Mag. loads. It also means they are very likely going to damage more tissue...." "....it’s clear the cartridge has the right stuff for coyotes, deer and even felonious fiends." americanrifleman.org, April 2010
"The 130-grain hardcast load from Buffalo Bore Ammunition is even suitable for bear defense."- americanrifleman.org, August 2018
"It will perforate a large bear's skull..." Buffalo Bore, of their .327 Hard Cast load
But when you compare the ballistics of these two very similar cartridges, ballistically, the .30 Carbine does roughly at 125-150 yds. what the .327 does at the muzzle.
I think it's pretty well established that the .30 Carbine's "problem" is the FMJRN bullets that have always been loaded in it. Load it with expanding or frangible bullets and I think one reaches an entirely new dimension. I'm not a big fan of gel testing bullets to determine their effectiveness, but there are a few YouTube videos of .30 Carbine tests with 110 gr. SP's and all of them expand well and penetrate quite remarkably considering their lack of sectional density.
I keep one of my carbines in the house for SD.
35W