Well for home defense a carbine is always going to be better than a handgun. As a former Marine I am a bit surprised you didn't come to the same conclusion. Loaded with 50 or 45 grain varmint bullets the penetration and ricochet potential of a 223 is FAR less then a 9MM Pistol. The same can be said for the fastest light weight 22 LRs.
And as far as her being to small and weak, I'd say give her a chance with an AR and see if that's really true.
I trained 3 kids last year on ARs. The smallest one was an 11 years old girl and weighted 67 pounds and they all did great. So how small is your girlfriend?
Here is a simple test .
Put out a gallon paint can or milk jug at about 10 yards. Giver her a hand gun. ANY hand gun. Even a 22 auto.
Have her fire 5 hots at the can and time them. No shots count that don't hit the can.
Now do the same test with a 10-22 Ruger, or an AR 15.
I think the results will speak for themselves.
Now if you want to go to the next level, suspend the can on a string between two poles and let it swing, so it's a moving target. Do that at only 5 yards. It need not even be swinging fast.
Have her shoot any handgun and then try a 10-22 Ruger and an AR15.
Don't believe me. Believe the test. You'll see.
If she is not carrying the handgun on her body , (if she has to go pick a weapon up,) there is NO situation where a rifle is not going to be a better weapon than a hand gun. Handguns are best for immediate reactions, and long arms are best for responses in a few seconds more. And remember, it's 100% OK to pick up both. A handgun in a paddle holster or even one to tuck into your belt is OK to have on you when you are going to fight. I always did so when I was a Marnie. But get the rifle first!
I have been shooting since I was a pre-teen and I am an old man now. I also was a Marine, and "combat skills" was one of the subjects I was responsible for teaching to other Marines, as well as DOD personal for many years. I have been doing it now for nearly 40 years. I can hit the swinging paint can with all my pistol and revolver shots, but it's still easier, even for me, with a rifle. Especially if that rifle has a large peep sight on it. Or a low powered scope.
How much more easy is it going to be for a new shooter?
Do her a big favor and try my little test before you make a decision.
Body carry 24/7 is the very best, but it's something she will have to start doing and start training for, in a serious way. If she can't body carry a handgun (or won't) get her a rifle. Even a 22 rifle is FAR better than a handgun she misses with more than she hits with, and any handgun miss is a sever liability in any neighborhood.
In the vast majority of civilian shooting cases, the one that lands the first hit wins the fight, and in most of those cases the one getting hit survives, but knows as soon as they are hit that they can't really win, so they break off the engagement. This is not all-out war,and the crook has no supporting army and no supporting medical establishment to help him, not to mention that fact that what every he is after, from property -- to sex-- to control/power, is diminished, or totally removed as soon as they are hit one time.
Only hits count. A powerful hit is better than a non-powerful hit, but ANY hit is WAAAAAAAY better than a miss.
My daughter is "5 foot nothing" and weighs 115. She has an AR15 as her home gun. She hunts and killed deer and elk with her 257 Roberts and has killed about 6 of them on the run. Would not want to be a crook trying to harm her.
My Mom is 4' 10" and weighs 124, and is 88 years old. She has an AR15 as her home defense weapon. She doesn't hunt at all, but can hit the moving can about 80% of the time at 25 yards. Would not want to be a crook trying to harm her either. LOTS of other 88 year old ladies would be a better bet.
My wife is tall and slender, and she doesn't like the AR as much as she does her AK47, which she has killed deer and antelope with on several occasion including some moving shots. She carries a Kimber sub-compact 45 at all times, but in every instance where she has had time to grab her AK, she grabs it. She has killed only one varmint with her 45 and missed 4 others, but so far I have only seen her miss one coyote and one fox with her AK. All the other varmints she's shot at with her carbine, she's killed. (maybe 12)
Coyotes running away from her usually die when she shoots at them. How well do you think a crook would do trying to come towards her?
Those are the ladies I love and care about the most and if I believed that a handgun was in any way better, I'd teach the to give up on the rifle. But that's kinda stupid don't you think?
Can you show me one case where any military on earth issues handguns instead of rifles?
If the answer is no, ask yourself why.
Also ask yourself how many hunters kill their game every year with rifles and how many do it with handguns. Handguns are not even used as often as bows and arrows. Why?
Because they are the most difficult type of weapon to learn to use well, especially under pressure.
In order of difficulty to attain proficiency , from easiest to most difficult, we have studied weapons and fighting systems and come up with this list. It may not be exact, but I am sure it is very close.
#1 Light caliber rifles.
#2 Shotguns.
#3 Heavy caliber rifles
#4 Light caliber handguns
#5 Bows and arrows
#6 Heavy caliber handguns
#7 Slingshots.
#8 Spears and lances
#9 Long heavy knives
#10 Short knives
#11 Heavy swords.
#12 Unarmed combat.
Please, for the sake of your loved one, think about what I have written here.