I use a 5mm Sheridan Silver Streak pellet rifle and that thing is very powerful. I have owned for over 40 years I believe, and it still works great. When I was a kid I used to squirrel and rabbit hunt with it all the time. Shot a lot of bunnies with that thing.
I never heard of derringers called mouse guns. Pocket pistols, belly guns, a adies friend.. but I like that mouse gun description.