After reading this entire thread a couple of times I'm not sure if there was a factory standard or not and if there was such a standard where was the measurement taken from. So far it's been suggested the measurement could be from the muzzle, from the breech, and someone suggested measuring from the butt to the bead. I checked three guns last night that letter with double beads and all three measure 18" from the muzzle. Because all three guns have different barrel lengths (26"28"30") I have to assume that it's the relationship of each bead to the other that would be the most important dimension. I like double beads for checking gun mount but find them impractical for shooting as I try to focus on the lead edge of the clay or the head of the bird when making a shot.
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