Thomas, In Brophy's book Plans and Specifications of the L.C. Smith Shotgun. on page 157, dated 6-3-'36 gives figures for chambers including the .410 3" chamber. All taper from .013-.015 including the 3" .410. In another chart dated Dec. 21, 1907, the 12 ga. starts at .811 and ends at .797 at 2 3/4". So it would be fair to say that any L.C. Smith factory done chamber would be that same dimension of .809 to .811 to .795-.797., even for the 3". The only thing that we don't know is what the bore size is. If it is .729-.732 then everything has to be in the choke area being longer.