Glock compacts (the G19 and G23) are a handy size if you're looking for something a little smaller than a "full size" handgun, but trying to avoid getting so small that the damn thing gets hard to handle. They're light (about 2.5 pounds loaded), incredibly reliable, and if she sticks with the G19 (in 9mm), recoil shouldn't be a problem. Not particularly expensive, either... they usually run around $500.

I'm with Raygun on the pump-action here... there's a lot less that can go wrong with a pump, and they're a lot cheaper. Trying to use a pump-action shotgun indoors can be a pain in the ass, though... the one time I felt it necessary to find out "what the hell was that crashing noise upstairs?", I left the shotgun in the safe and brought along a .45. Turning corners or entering rooms with a three foot long gun poses some problems, especially in homes with a tight layout. However, if her plan is something along the lines of "hole up in the bedroom with a gun aimed at the door," the shotgun would be an excellent choice.

You can get a Remington 1100 for 700 USD, a 20" replacement barrel will cost you another ~180 USD. AS for a handgun, best get her to the gun store and try some out. Things like the size of her hands will matter in handgun choice. As long as you make a reasonable choice for the weapons, training time with them will be muhc more important than the actual choice of weapon.

Any brand name you recognize (Remington, Winchester, Mossberg) will get you a pump action that's just fine. Much like buying a car, after a certain point the name brand stuff just becomes a matter of someone's personal favorite.

One thing you might want to look into (depending on how your mom handles recoil) would be something like a Knoxx stock or reduced recoil ammo or something, though. No matter how scary a pump action is, and how high the damage code of your average 12 guage might be (so to speak), a miss is still a miss. Buckshot doesn't explode from the barrel in a cloud of pellets the way you see in movies and video games, with a foot of spread a foot away -- you still do have to point 'em in the right direction, handle the recoil, and hit what you want to hit.

For a back up gun (which is basically what you're describing), I think it'd be tough to beat a Glock. No safety, no hammer to cock, no nothing. If it's loaded, it's ready to go (and if you pull the trigger, it'll fire). They're also ridiculously easy to maintain (on par with that shotgun, in fact). My own preferences aside, the same holds true for most handguns as it does for shotguns, as mentioned above -- any big name brand will probably work about as well as any other. A lot of advice comes from personal preference. Stay way from stuff like ARs and AKs for your average home defense gun (unless you live out in the middle of nowhere, with no one else in the house with you) -- they'll blow through interior walls disturbingly well, even after having blown through the bad guy.

For myself, I'd go for some flavor of Remington 870. More cops use 'em than any other shottie (though it's true Raygun's Mossberg pick is the military's favorite), and they're just dirt simple and reliable. IIRC, Remington's also got some sort of $30 back rebate gig going for the month of October, too.

Your best bet is to drag your mom up to a gun store and ask her what she thinks, too, though. While getting a firearm as a gift is always awesome, they're also very personal decisions (arm length, cheek weld, ergonomics, all different from person to person), especially when it comes to something as serious as a home defense/personal defense weapon. Haul her up to a local shop or a gun show or something, and have her see what she likes the feel of.

There's a lot of good info on home defense shotguns, but I noticed one thing that hasn't been mentioned yet.

20 gauge shotguns.

Less recoil, ammo also readily available, and still sufficient for home defense.

It might be worth trying one before you settle on the 12 gauge.

I can't believe China allows access to this forum.

Shhhhhhh!!!

No serious defensive firearm/cartridge combination is going to be bothered by light internal walls.

Le Mas Limited seems to have gone under and there's no mention of blended metal projectiles anywhere that I can find from the past couple of years. So much for that.

I met a guy who shot a burgler in his home with a shotgun. He told me he had always thought a shotgun was the best home defense weapon. After cleaning up the mess from that burglary he said, "If I had to do it again I'd use my Glock." Something to consider.

Thanks for your insights, guys. Seeing as I'm in the FSM right now I can't exactly take my mom to a gun store, but I can forward the suggestions along.