I got stuck on shotguns because people keep making them sound like the best thing since sliced bread. Plus, they're very inexpensive, effective, and she liked my 20 gauge.
They're nowhere near the best thing since sliced bread. Shotguns are especially good for home defense for
two groups of people:
1) People who
don't own anything else that would make a good choice (that
anything else would include high-power rifles for which lightweight varmint rounds can be found- any high-power rifle with lightweight expanding bullets will decisively stop a human threat at close range, while offering minimal penetration).
2) People who already shoot shotguns for sport so much that their skill set with a shotgun is already very highly developed.
Let's put this into perspective.
Dave McCracken, the patriarch of the fighting 12 gauge shotgun here and at TFL, was building an AR15 when cancer took him much too soon.
Pistol caliber carbines, however- when the calibers are the service calibers of 9x19mm, .40 S&W, and .45 ACP- have their own significant disadvantages. Most significant is overpenetration, and people who think simplistically automatically seem to equate larger cartridge cases with additional penetration in structure, which is frequently
not the case. In most cases, high-speed, light-for-caliber projectiles tend to fragment much more readily than slower, heavier-for-caliber (including handgun) ones.
You're going to have a hard time finding an "ideal" HD piece and still being inexpensive. Some downsides>
PCC: overpenetrative in structure.
Shotgun: heavy recoil. Overpenetrative with 00, and underpenetrative with birdshot (my suggestion of BB is still not enough penetration for most experts, and then the other "any shotgun shell will work" extreme screams at me for saying suggesting light birdshot is reckless
).
AR15 and similar: high cost.
Single-shot: reload speed.
.22LR Rifle: lacking in stopping power
per round.
So, your friend has to pick the liability she thinks she can most live with. The truth is, the biggest difference is not between any of these choices, it's between having a firearm and not having one. Even a .22 single-shot could be the difference between life and death, even though it's just about the last firearm I'd suggest for HD (just after a pistol-gripped 12 gauge shotgun).
If your friend is willing to practice with it, a 20 gauge loaded with Foster slugs or 0 or
#1 buckshot should work fairly well,
though all of these will recoil more than that SKS.
Hope that helps.
John