Rowan will make you a bespoke adapter bored precisely for the OD of your barrel, assuming you can measure it with some precision.

My barrel was not truly round, and had a major and a minor diameter about a thou different.

Rowan make it a very slight clearance fit so that any corrosion will be unlikely to fix the two parts together. In my case I wrapped a layer of PTFE tape around the barrel and made it a slight interference fit. That gives the grub screws less work to do under normal circumstances.