45-70 in a derringer!
Heck, if you are going to do it, go big... after all you do have two hands and two barrels; after shooting one barrel per hand, you are done! No need of additional rounds, as you don't have a third hand to pull the trigger even if you did.
Discussion of said piece:
I kind of ascribe to being able to accurately fire and control any firearm for more than two rounds. Personally have .380 in LCP as my "mouse gun". Any larger cartridge goes in a larger firearm for me.