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Except in the case of something like a revolving rifle or recessed breechface; make sure the breech is closed, then use a rod to measure from the breechface to the muzzle.
Most of the time they go by whatever the manufacturer says it is. The actual measurement doesn't matter. If Ruger, for example, says it's 4", it's evil. The cylinder doesn't count in any type of revolver. Rifles included.
