Normally you would use steel wool. A simple easy way to get measurements is to find a lead ball 10 to 20 thousands larger than the rifling put 20 grains of powder drive the ball in to the barrel and push it all the way down on the powder and shoot it into a large wad of cotton batting with a card board backing 4 or 5 inches thick. When you retrieve the ball your measurements will be the same as if you got them the hard way.