I've carried the .22 LR mini-revolver for years. It's a decent little gun for deep, deep, deep concealment when there is no other choice. It's also very safe and reliable. As for efficacy, a small caliber to the face will ruin everyone's day, even a .25. It's an edge.
.22Mag will put the hurt on somebody with a quickness.My father carries a ruger blackhawk in .22mag.I've seen him shoot groundhogs and blow the top of there heads off with it.I got alot of respect for that little round.the .22mag and .22 hornet are two of my favorite calibers.
The reviews on NAA 22LR vs NAA .22 mag that I read suggested that the mag was not a significant increase in delivered energy. I bought the 22LR and it is a surprisingly neat bit of hardware. It's easy to misplace so make sure you put it somewhere consistently. Don't ask me how I know this.
I believe that you are going to have quite severe velocity loss out of that stubby little pipe as opposed to a longer barrel. Most of the powder is going towards making fireworks like prprd_1 referred to.
Unless you're hoping to hide it in your hoohoo, I'd recommend something in .380 (only for the purposes of concealment). Kel-tec makes a .380 that is less than an inch in thickness and not much longer than the gun you're talking about. It is a good deal taller, but I can't imagine it will give you any trouble with concealment.
I'm not personally a fan of Kel-tec, but I think I'd trust this gun more than I'd trust a .22mag with a fingernail sized barrel. If the .380 is still to large, they make a .32 that is smaller and will still do more damage than a .22. See the following site for pictures and a review: http://www.ericsiegmund.com/fireant/archivesmt/003394.html
The mini revolver is good as a second or third weapon. I would not carry one as a lone weapon however. My summertime carry gun is a PA 63 and the back up is a spare mag and an outdoor edge folding skinner.
I have the Kel-tek .380. Bought for my wife. Carry hp starfile in it. It has quite the muzzle flip. Very easy to hide. I also carry the PA-63. Due the lightness of the gun it also has some nasty muzzle filp. Never seen HP in 9x18 so I have fmj. Also easy to conceal and you can get it about $150 bucks. I have larger more expensive hand guns but it is nice not to have to worry if they show or not. Many say carry at least a 9 and a rut sak full of ammo for a trip to store. I like to just throw it my pocket and go. I like them both for different reasons. The most important thing about buying is getting somthing you like, feel comfortable enough with it that it is almost instinct.
The PA63 is a neat little handgun. I keep mine loaded with FMJ because it works (so far 100%). Yes there are hollow point rounds but they are a bit pricy. My wife carries a M&P .40 cal but I like small guns better.
My buddy who owns the gun shop i shop at talked me into getting the mag vs. the LR. His red neck ballistics tests were as simple as the 22lr didn't go through a 2x4, and the mag did. haha...........really though