I had a German sidelock with .020 near the muzzle. It was a 1912 or so gun and probably had the barrels cleaned up at least once. With 2 1/2" cambers 16 gauge and weighing on 5lbs 10oz I was using RST low pressure loads to shoot it and used it for several years for desert quail. Did shoot a couple rounds of skeet with it but it was still a nasty recoil for target work. So think .020 is okay as long as that is just near muzzle.