41 RF caliber. Interesting thing is that these two derringers are only 11 serial numbers apart. Most are around 100 numbers apart. They are fitted in a nicely done Moroccan leather case with a buckskin interior. French fitted. The guns are in very fine condition. One has about 80% barrel blue and the other has about 40% with the balance turning a plum/blue color. The one with the most blue has very little silver on the frame. The one with very little blue has most all the silver on the frame thinning in high spots. Excellent fancy grips. Mechanics are good with bright shiny bores. Very nice pair of factory engraved Colt derringers. They are nicely fitted in case with a period screw driver and seven rounds of ammo. Original throughout.