Mike the cylinder chambers, not the barrel
DrLaw was suggesting you measure the diameter of the cylinder chambers (not the barrel bore) to help decide what size ball to use. Black powder revolvers use round balls slightly larger than the marked barrel bore (caliber) in order to engage the rifling (single shot bp pistols and rifles use a cloth patch around the ball to achieve the same result). .36 cal revolvers all use either a .375" diameter ball or a .380" diameter ball depending on the manufacturer and type of gun. Some can use both. Unfortunately it's often not possible to determine the ball size in advance, even knowing the manufacturer, hence the suggestion to measure the cylinder chamber diameter.
You can also get a feel for whether you have the right size round by loading it into the cylinder chamber. The correct size will take some force to press home and it will shave a small ring of lead off the diameter.