it's unlike a rifle, with grooves deep enough to turn a tight fitted patch or brush, and let you gauge the distance required to complete a revolution.

i can't reliably stay in the grooves, so am reduced to looking through the bore. as best i can tell there is a bit over 1/2 of a revolution in the length of the bore (70cm) - certainly no more than 3/4.

"it's a poor sort of memory that only works backwards."
lewis carroll, Alice in Wonderland