For back up, I carry a couple of North American Arms mini revolvers in 22 mag.
You can get them with both 22 mag and 22 long rifle cylinders.
I do a lot of shooting with the long rifle cylinder in them.
With the Hornady XTP 22 mag, you get the best MV that you will find in the 22 mag from a short barrel. That is what they make them for.
I would suggest the North American Arms .22 mag revolver. You can easily conceal it in your pocket and it looks no different than a cell phone when you are looking. You can get them in snub nose and like 1" or 2" barrels. I personally own the snub nose. I love it. Packs quite a punch in such a small package. Don't let these people tell you not to carry a .22 because you dont want to trust your life to it. Any gun being carried is better than no gun at all. I find myself wanting to carry that small revolver because it takes no effort whatsoever, and you dont even feel it in your pocket. Plus since it is a revolver if you get a FTF you just shoot again. Hope this helps.
Plus one for the Beretta 21A. I had one , but my daughter appropriated it, and loves it. Accuracy at 7 yards is very good, and it functions well. If one can afford it, a 22LR Walther PPK/S is outstanding, and good ones can usually be found on Gunbroker.