My 1947-vintage 310ASE measures .667" in the right tube and .668" in the left, with .018" choke in the right and .030" choke in the left. My 12-gauge 310AS from 1918 measures .727" right and .728" left. When I got the gun it had .038" choke right and .044" choke left. Kieth Kearcher opened it to .010" and .020".