I basically use Beagle's method. Last week I encountered a barrel that had no identifying marks. I could not even venture a guess. I even had to measure the bore to determine that it was a .45. I marked the rod in the normal manor and had to go for a half rotation. Then multiplied by 2 and came up with a 1:44". Had to check it three times to confirm and heck, close enough to 1:48" for me to work with. I was really hoping for a 1:60 or 66. Ya know what they say about "hoping" in one hand and; ......Yeah, you might have to do some math. I didn't pay attention to the part about the 20" barrel.