If this were my shotgun, I would not alter the forcing cones, I would just shoot 2.5" shells in the 2.75' chambers and not worry about the configuration of the taper of the cones. Have you had the NBD measured, as well as the choke tapers? Many European 12 gauge shotguns, especially pre-WW11, ran closer to a .7250" NBD, whereas our American 12 gauge shotguns were closer to a .7300" NBD--Do you have period buttplate for this Simpson, or are you going to install a pad for a longer LOP. Nice clean lines, and good to see the cheekpiece sans the usual ruptured eagles and leaping unicorns the German stockers liked to deep carve into such areas--
