long ago and far away, Brownells used to carry something very similar. It also had a long rod added that laid on top flat of barrel to get the distance from muzzle too. Had a flat plate or block on the muzzle end, with a short brass locator pin that went into the muzzle. Last I saw was in Ralph Walker book "Black Powder Gunsmithing", about 1979? Shudda bought one then. Now I gotta make one too.