can we see your set up to better understand what your dealing with?
theres about three ways to skin this cat, but your shop dynamics are gonna dictate that
When I do longrifle barrels, I support them on two rear rifle sandbags The leather v type. They can be rolled and slid back and forth and still stay under the scope. Engraving curves does take some extra wrist action. I suppose if a lot of rottion is needed, one could place a board or plywood on a turntable and then put the sandbags on it. With the length of a rifl barrel it is a matter of adapting to conquer.
Jeff, how do you use the fixture? I see the barrel supports but then what? I do a lot of 42in barrels JJ's post vise in the middle of the floor and walk around the barrel works best for those but still not great.
In the picture you will see the leg that comes down, it has a wheel(caster) on it. You would have to sit your vise away from your desk so when you turn the vise the barrel will follow on the wheel.