How do you measure a firearms barrels length?
For a rifle or shotgun-
Make SURE the firearm is unloaded, and action cocked (so that
firing pin is not protruding from face of bolt). Insert a cleaning
rod into the barrel, mark rod at the muzzle, withdraw, measure from
tip of rod to mark.
Pistol- same way.
Revolver- Hold a flat strip of material (thin metal works)
across the rear of the barrel (start of the forcing cone) Insert
rod into barrel from the muzzle, slide until it touches the
material. Mark rod, withdraw, measure.
I DID say make SURE the firearm is unloaded.