I too think the calibre is in the 360 BPE class and with the 8.7mm diameter I'd guess the bullet will be closer to 0.355" diameter. I think that the reason for the land to land diameter is that after a pilot hole was drilled, the tube was presented to the proofhouse in a state just prior to being rifled, possibly after being lead lapped where a small rod would have been inserted and a lead cylinder poured and formed to the tube diameter, and the diameter is still akin to the plug gauge diameter but in mm. If indeed the lapping was performed, they may have had pre-made cylinders. The diameter is still for the pre-rifled diameter and the pre-rifled tube was proofed with a solid projectile 0.2mm LESS than the diameter of the tube, with the rifling being cut at a later time with the exception of the "Crown" over "B" scenario where the arm was proofed in the final state. But the maker had to provide some direction as to the final cartridge choice as the proofloads are dependent, i.e. multiples of the load of the cartridge selection, on the load data of the desired cartrige. Being purely conjecture, I wonder if some of the tubes were branded as being one of most common and more stringent of the 360 BPE loads, proofed under that load and then subject to a slight alteration near the end. Or if the cartridge selection was set in stone at the beginning and there wasn't a deviation. This would all be pre-1939 as post 1939 the rules called for the tube to be pretty much near the final state and the cartridge to be selected.
Kind Regards,
Raimey
rse
