This is a French short range adaptor for a 12 Ga shotgun.
The caliber is 22LR
the top view shows it open in order of removing the fired case.
the bottom vue shows it closed, ready to shot.
On the side it is written :
Mousseaux & Erfeux
(Bte SGDG means : Patented, no warranty of government)
The total length is 220 mm (8 5/8)