Good advice above.
I use a jag and patch which can be reversed in the barrel. Make your 12 o'clock mark on the handle end of the rod. Get it started in the rifling and make a mark at the end of the receiver (or muzzle if you go from that end). Mark it again when the mark on the end of the rod has made one complete revolution. Remove the rod and measure between the marks.
I do it this way because the index mark is always visible and hasn't (usually) passed my reference point on the rifle.
On some you have to measure a half a revolution and multiply by two... My 1 in 38 Marlin comes to mind.
Proud member of the NRA and Texas State Rifle Association. Registered and active voter.