First of all, thanks for all the responses, I never expected this many suggestions within a few hours of posting
I am greatly appreciative of all the help! I am trying to help my girlfriend choose a gun for home defense and while some of the suggestions are similar to thoughts I had, some are things which never crossed my mind. I will try to address each response.
ScareyH22A - She would prefer not to have an auto, as she is a firm believer in Murphy's law (what can go wrong... will go wrong). Than you for the suggestion though, if I can get her into skeet shooting (VERY likely, she's a keeper!) we will be looking at autos for that. Also, do you happen to drive a Prelude?
Ohm - I had forgotten about the SpecOps stock. I would like one for myself, but she has set a firm price limit of $300. I may get her a stock as a gift, but we'll see if she keeps the shotgun for bedside duty first. She turns 21 in June and has already taken her CCW class, so she will be carrying a pistol then and it's possible the shotgun will find itself in the back of a closet at that time if she's more comfortable with a pistol.
xx7grant7x - I am taking her shooting this Saturday with a 12ga and if the recoil is too much, which I suspect it may be, we will look into a 20ga. I'm not as comfortable with the idea of a .410
c5_nc - WalMart's in my area do not sell guns, but I will try to find one that does.
BhmBill - Thank you for the suggestion, I certainly think it is a good one. However, it is out of the budget for now.
Iansstud - I'm taking her shooting with a 12ga this weekend, I have regular 00 buckshot, still looking for some low recoil rounds to try. Going to look for a 20ga pump if the 12ga is too much.
Lee Lapin - Thanks for all the suggestions. I will ensure the gun we choose fits her well and I will train with her as often as time and budget constraints allow. She is very into guns as well so I don't think this will be an issue. As I've mentioned in responses to other members, I'm taking her shooting this weekend with a 12ga, but it has a normal length stock. I'm hoping she can shoulder it safely, but if not I may pick up a 12ga and 20ga youth shotgun (one of each if I can find them) and keep the 12ga if she doesn't like it. It seems a youth shotgun would be more ideal for clearing a house than one with a 26" barrel and full length stock, and if never hurts to add another firearm to the collection. My girlfriend read your post with me and she had a huge smile at the thought of a table full of shotguns for your niece to choose from
Hungry Seagull - I'll keep an eye out for one, there's a pretty good local gunshop that may have one in stock, and could certainly order me one if that's the direction we go.
Old Guard Dog - I'll look for a box of this ammo before we practice this weekend.
Jubjub - Thank you, I believe that's one that Lee Lapin was referring to.
JImbothefiveth and JShirley - A rifle I would trust in this duty is out of the price range at the moment. $300 is the upper limit for what she's willing to spend on a shotgun as this June she'll be in the market for a ~$1,000-1,200 pair of carry pistols (main and backup gun).
Again, thank you for all the suggestions, I am very impressed with the hospitality here