To add to all of what has already been said about size restrictions. If you want to go semi-auto your only choice is .22lr and the most popular choice there is probably a Browning Buckmark of some sort, likely fettled by Westlake engineering.

If you go down the revolver route the most common are .22lr, .38/.357, .44 Mag but you could probably find some obscure items like .45ACP with half moon clips if you try hard enough. Lever action guns tend to be more popular as does reloading for the centre fire calibres.

Paul.