5/8 barrel I.D. needed for ballistics test.

Where are you located? What is the required burst pressure or what is the the final muzzle velocity? You might get away with buying a shotgun barrel and custom reloading shells to a specific power level.
Andros Manufacturing in Rochester NY does machine work like that.
I also know of a mil contractor that does custom reloads to match projectile velocities complete with chronographed data for your projectile. (I have used them). They are a little expensive but well worth it when you consider the convenience. ~$35–50 per round. But they are perfect and fit right in specified shotgun like a standard round. It does not get any easier.
(You have to be a US company due to ITAR rules. PM if you need info.)