It is an English pattern probably in what they called .320 which is a black powder round.
It would be more of a personal weapon from the Great War and they stopped making the ammo commercially during the depression.
It had a muzzle velocity less than 700fps, so a real show stopper. It came in a variety of calibers but all were black power and anemic.
The loading gate on the right would be opened, you pull out the center pin and swing it over in alignment to the cylinder and push out the spent shell adding in a new one in it's place.