I personally carry a ruger ACP .380 in the summer. Its reliable, I can shoot it dead nutz target shooting, its small, and its light.
You will get many opinions here, Some will toat around a 44MAG just to feel as they have the biggest baddest gun! My opinion on a carry gun are in this order.
1) Reliability (buy a well known top brand name) When I purchase a gun I will buy 100 rounds of 3-4 different brands of ammunition from personal protection to target cheapos! My test is that atleast the personal protection ammo should be able to rapid fire 10 clips without one error or fart! I will accept one hiccup jam mis fire from cheapo ammo. If it does fail on ANY personal protection ammo such as Cor Bon +P, Remington, or Federal Hydra Shocks I will NOT carry that pistol for protection.
2) Accuracy: In the majority of using lethal force to protect yourself it will be or should be within 15' You should be able to fire your gun at 15' center mass without no problems what so ever, cause your not going to shoot as well with adrenaline flowing to survive its a who different ball game once you make the decision to use deadly force to protect your life!
3) Size and comfort and how many options do you have with holsters. Do you want to carry on your ankle. inner belt, shoulder?
I am happy with the lil ACP .380. Yes some may laugh, its a girls gun, its to small, yadda yadda yadda, but i know when I carry it, its going to go BANG when I pull the trigger, I forget its on me, and that I can hit deadly with it.
A .22 caliber is a very deadly round since its known to ricochet in the body off of bones damaging vital organs, the stopping power is different.
Good luck in your choice.