As the man said- rod with a snug fitting oiled patch. Insert rod all the way. Make a "flag" of tape wrapped around rod AT THE MUZZLE, with little flap sticking out. Note where tape points. Withdraw rod, allowing it to rotate. Note when tape has turned 1/2 turn, mark rod at muzzle. Measure the distance, and double it. (for modern guns, just measure distance for one full turn, rate of twist on muzzleloaders is too slow to measure a full turn, so measure half, and double it).