37mm Auto Cannon System McClean ("Maklen" in Russia)
One of the first if not the first gas-operated autocannon. Built by the same McLean who did the original design work on the Lewis Gun, this weapon was built in the standard 37mm, 47mm & 57mm calibres although only the 37mm saw use as far as I can find out. Used only by Imperial Russia and everyone in The Russian Civil War and the wars at that time on Russia's periphery. In Russia it was called the 'ZTD Maklen Gun' and used on ground mounts and on armoured trains, usually on improvised mounts.
This weapon did not fire the ubiquitous Hotchkiss 37x94mmR round but the far more potent 37x137mmR used by the US Navy and could perforate any 1919 combat vehicle out to 3,000m. It was fired from five round clips at either single shot or 100rpm.
As far as I know the only existing weapons are all in Russia.