There's FAR more to it than that Tiriaq....... Just from an Armourers perspective, we'd re-barrel one body with, say, 25 barrels to choose from and they'd all under or even overturn differently and even when it was all correct on the parallel gauge, the foresight block would/could be visibly off line and.............. and.............. Someones STILL got to set the machinery up and if they're doing it from drawings gleaned from back engineering, well.........
Just mark my words............. If they're serious, ask them to speak to someone who has breeched up a few or even had cause to manufacture just the breech end. And at the end of it, you've only got .010" CHS to play with unless you supply a set of bolts.
Not a job for the faint hearted. Just phone me and remind me of my bet at the end of December and we'll down a small scotch together over the phone!
Added later. Errrrr, I meant bolt heads and not bolts! Sorry