Like what you are doing. I've an old English double 410 by WJ Jeffery. Barrels very pitted and one day it will become a double rifle. Given the size of the action will you have enough wall thickness to withstand the pressures of the 6.5x57r ? Did you think about doing a shoe lump joint rather than a mono block so that you can keep plenty of barrel thickness. This is the approach Heym take - http://www.heymusa.com/heym_tech.htm. No criticism implied or intended - just a question.