I agree with Erik...
I have owned two "Mouse Guns"...a NAA Mini Revolver in .22 WMR with the .22 LR convertible cylinder and a Beretta 21A .22 LR. I sold both as I don't feel that they are size efficient by today's standards. (Although they might have been at one time in the past.) I have three small guns that will fit in my back pocket or on my ankle and shoot serious cartidges...a S&W 649 Bodyguard .38 Special, a S&W 638 Airweight Bodyguard .38 Special, and a Kahr MK9 Elite 98 9mm. I see no reason to go smaller than these guns. I do have an American Derringer Model 1 in .45 Colt/.410 that I have just because it is cool, but not because I think it is a good self-defense weapon. If I was going to consider buying another "Mouse Gun" I would probable look at one of the new NAA Guardians in .32 ACP or .380 ACP, although I have no personal experience with either. I believe that the .25 ACP is a worthless cartridge. It offers nothing not already covered by the .22 LR, .22 WMR, or the .32 ACP. I will say, however, that I would not look forward to being shot by one, or any other gun for that matter..."Mouse" or not!