Two part barrels may be going out of fashion, but I just found a great tool. Alexen Model produces gun barrel grinding tools for multiple scales. It won't leave a flat spot like sanding and would cut down on the time you spend removing the seam. You can read more here:
https://www.scalemates.com/companies/company.php?id=7478

i have few of the voyager scraper tools in different shapes like this one...



they have i think 4 different type,handy also the nuts maker come with them