Using a good swivel handle cleaning rod like a Dewey - I have a mark indexed on the rod, draw it through with a tight dry patch in the holder till the mark is at the muzzle, rotate the barrel so the mark is at 12 0'clock then continue to draw the rod till the mark presents at 12 o'clock again and measure the distance from the mark to the muzzle.
12 inches? I pulled 12 inches through to get one complete "twist" of the rod. 1:12.
I do this a couple three times to make certain that there is little or no slippage.
I've also done it carefully at gunshows with non-swivel rods as I don't take my Deweys out of the shop. A little more patience and finesse is needed but they've been very accurate as well. On these, I stop the rod where I want, run a ring of tape at the muzzle on the rod, mark and go...
