12 ga, stock was changed sometimes
http://www.gunstar.co.uk/wilh-collath-slide-action-12-bore-gauge-shotgun/Shotguns/738585

Wilh Collath Slide Action 12 Gauge

Extremely rare in the UK, Wilh Collath of (Söhne), Frankfurt in Germany.

One of leading makers of rifles, sporting guns and proprietary cartridges (Tesco Geschoss).

Collath succeeded Georg Teschner in the 1890s. The company traded until the end of the Second World War. Eccentric Lock action Collath.

This firearm was made between 1920-1939. It is completely original & in great condition. Was brought from Germany to Canada ( dont believe the gentleman pay for the gun) after WW2 & was then brought to N.Ireland in the 1980's.