Aaron McMichael machined me a nice, heavy base for my old Italian engraving vise. I have it mounted on a GRS turntable and it works really well for hammer & chisel engraving under a microscope. I would imagine it would handle the heaviest of gun barrels or anything else that could be thrown at it!
He must have one monster size lathe to turn that base!
It makes me very nervous seeing your hammer anywhere near that beautiful scope. Even in those skilled hands of yours!
<sigh> Well, that just turned off the tape running in the back of my head saying "where can I get a vise that will turn in the center and is self centering... where can I get a vise that will turn in the center and is self centering...." and ended the fun of searching the world for just the right vise....
Just mount it on a big hunky cylinder, put the cylinder on a centered turntable, done. Some people just take the fun out of everything by thinking about it a different way.
I wonder what you'll put that big hunk of steel in so you can engrave it? It looks awfully plain. I bet it will get at least some embellishment one of these times.
I was just looking today at two Italians website where i see some pictures using vises like this, but mounted in a different way , your base look great and is a very interesting setup!