Derringers, usually 2 shot. IMHO, anything more than a .38 SPL likely has way too much recoil to even use the 2nd shot. The .38 might also do this. Perhaps .22 LR or .22 WMR are indeed the best?
The use of wax or oil coated bullets (.22LR Match ammo) is likely the best as it will drag and deposit bacteria and dirt into the wound. This will not immediately fatal, makes for a very difficult wound to clean. The fewer witnesses at your trial, the better.
Smooth, hard sided bullets are less prone to this 'dirty" ammo scenario.