If you have a good quality cleaning rod (preferably one which the handle rotates or spins freely), put a nice snug patch on, then at the handle end take about a 1/2 to 3/4 inch long piece of tape, fold it in half around the shank of the cleaning rod leaving the ends sticking up. Index the tape "flag" to top dead center as you insert into bore, push the rod letting the patch rotate it with the twist of the rifling, until the tape "flag" comes to top dead center again. Measuring the length of cleaning rod inserted will give you the inches in one complete turn of the rifling. (Example: 1 turn in 9 inches, 1 turn in 10" etc)
Using care this method is really quite close. Hope this helps.