I am confused by your (Bowen475) statement that Greener implied that your gun was made as an eight bore. Did they say it was an eight bore or not? Weighing the gun may tell you a lot. Even a light eight bore should be pretty heavy. My 1885 Greener Royal eight is Nitro Proved for 3 3/4" shells and 2 1/2 ounces of shot. I am not sure of the wall thickness, but it is a heavy gun, around 13 or 14 pounds. If we knew your bore dimensions and wall thickness, we may be able to give you a better idea of it's suitability for firing with heavy loads. I am much more trusting of my own wall thickness measurements than I am of a stamped proof mark. I would not in any event send a gun back to England for reproof.
