DT,I hope no one will try the "radical bisection" unless they have some of those useless old Purdey barrels lying around, in which case I suppose it would be OK. Bore gauge with graduations on the stem is said by Niklas to work; wall thickness gauge with similar graduations or perhaps a single hashmark representing the expected beginning of the change in gradient works for me. Whatever happened to the "mad biker" Wilhelm Gregor and the 20 hull in the 12?

jack