The operation shown above was done on my lathe with the 4 jaw. It was fun. I haven’t had that thing out from under the lathe in far too long.
Double checked my mill with a dial indicator and my X axis is off over .001″ per revolution of the handle. My holes are off further than that so it is partially my fault as well. Enough of this mock up works that I can feel confident making some adjustments and building a second. I’m also toying with the idea of a second piece bolted to the front of the trunion to hold the barrel in place. Then the barrel would not need to be pressed in and it could be replaced easily if needed.