The smallest I know of is the NAA mini revolver. Comes in .22LR and also in .22Mag. I've shot both, didn't hit much with either one. They hold 5 shots. Seemed well enough made, and interesting to shoot just so I could say that I did. At the distance you could hit something with one of these, you could also stab it, and a knife would be more effective IMHO. But if you want small ... this is it.