You are a very good tool and fixture designer, as well as a top flight engraver, I like the look of this arrangement.
I am a babe in the woods when it comes to long barreled engraving, so this is only a simple observer's question, might there be any advantage for the gun engraver to introduce another degree of (controlled) freedom. For example, with this very nice counter-balance design of yours, could you envisage the barrel not being held directly locked in the ball vise, but instead, your device being held in some way in the ball vise, allowing one of the barrels to slip into a fitted shaft that allows the barrel to be revolved safely upon its own axis, with your spare hand, as engraving takes place?
Hornady already makes a whole bunch of modified cases with the primer pocket drilled out and tapped for their overall length gauge. Perhaps if you were to use the same thread as those, it would make that part a lot easier. Just an idea.
"a 5/16x36 tpi tap." boy, they weren't kidding when they said a 'special extra-fine thread'- that's special & extra-fine alright! that's half again more tpi than a standard 5/16-24 UNF. (on an interesting related note of trivial import: did you know MSC sells taps for 1/4"-18, 20, 24, 27, 28, 32, 36, 40, 48, 56, 64, & 80tpi? and all those PLUS a 72tpi in 5/16"? personally, i think that's just asking for fights between design engineers & production engineers...)