I would venture to say that most people discount the usefulness of a derringer either based on hearsay or a bad experience with an inferior gun or anemic round. Since it seems in vogue nowadays to say that a .32 or .380 is a viable pocket pistol, I would say that a .45 colt or .410 000 buck will do significantly more damage to a BG than one of these normal mousegun cartridges (For God's sake, people actualy look at the NAA mini as a better pocket gun!!).
I find my derringers to be as reliable as a revolver (can you truly say that about any mouse auto?) and packs a very potent punch at the intended range of 3 - 7 yards.
I have literally dintegrated melons and other targets with my derringers. I can also hit targets reliably out to 7 yards or so.
So - I would not in any way prefer a knife to a derringer for self defense. I carry one one of my derringers everyday and trust my life to it's 2 powerful shots.
Just thought I''d stick up for one of my favorite handgun styles...
Chris Canis