This is a test of commonsense. Complete the next sentence:

How to measure gun barrel
Unload the gun.
Open the cylinder and remove any cartridges that are still inside.
Point the gun in a safe direction and cup your left hand around the gun, keeping your fingers clear and away from the muzzle.
Push or pull the cylinder release latch on the left side of the gun with your right thumb. Swing the cylinder out to the side of the gun.