A proven ring of .835 to .865 would be suitable to zero the one head Galazan gauge for eight gauge and big ten gauges. The biggest ring that comes with the gauge is .775, the nominal ten gauge bore dimension. .835 would be the nominal eight gauge bore size. I measured a cleaned up eight gauge this weekend by checking the gauge at .835. The bore measured .900 on the gauge, but I'm not sure the gauge is accurate above .775. I need a bigger ring to check on that. Whatever you can do would be appreciated.