400NitroExpress

" I did not include re-blacking the barrels because it usually isn't required. It wasn't the last time J.J. re-regulated one for me."

Could you explain how you re regulate a double rifle which generaly means unsoldering the barrels at the muzzle
and resoldering them, then firing new loads means that you do not have to reblack the barrels ?

Re re chambering and reproofing
IMHO, A re chambered double rifle is worth nothing if it has not been reproofed.
It also means you can only sell or hunt with it in a few countries.
My recommendation is to get it reproofed in the original country of manufacture.

500 Nitro