The 12mm Lefaucheux is a metallic center-fire cartridge.

It was originally created as a rimless pinfire cartridge using black powder employed by the French navy on the Lefaucheux M1858 revolver.

Later it was adapted to a center-fire cartridge by the French Army in 1873 for use on the MAS 1873 revolver.

Originally, the 12 mm, was a classic Lefaucheux cartridge, with a side pin from its introduction in 1858 until 1873, when it was adapted and became a rimmed center-fire cartridge for use in the MAS revolver 1873-1874 then used by the French Army.

[1] These are the characteristics of the '12 mm Lefaucheux' cartridge:[1][2][3][4] These are the most frequently encountered ("headstamp") tags:[3] In these markings, the initials "G. E" referring to the manufacturer "Ernest Gaupillat", and the other letter and digits referring to the batch (probably place and date).