With guns that have a slower ROT, you won't get a full turn with your mark on the rod. Ex: a 1:48 ROT, common in both .45 and .50 caliber barrels, is used in lots of barrels that are only 28-32 inches long.
Tape the rod, at the muzzle, after inserting it down to the breech, with a cleaning jag or brush, and an oiled patch. Put the tab of the tape in line with the front sight, as a index indicator.
Now, pull back slowly on the rod,turning it counter clockwise with your fingers, so that the patch stays in the grooves. You may get 1/4 or 1/3 or 1/2 turn in the barrel, depending on its length. You will have to eye-ball where that tab winds up. Redo this measurement several times to learn how delicate a touch you need to get an accurate reading.
Most people, doing this the first time, hold the rod too hard, and get all kinds of short measurements! I always suggest that you try this first on a barrel where you know the ROT already. When you have perfected the technique, Then use it on an unknown barrel.
The math is fairly simple. If you get only 1/4 turn, multiply the length of the rod, from the muzzle to your tape, by "4" to get the actual full ROT. If you have 1/3 turn, multiply that measured length of your rod by "3". If you have a 1/2 turn, multiply by "2". :thumbsup: