Frankford arsenal makes a tool that attaches to your cleaning rod you close the bolt(action) run the rod down to touch the bolt face install one end, then open the action place the bullet you want to use in the chamber and hold it into the lands with a pencil or dowel then run the rod into the barrel to touch the bullet and attach the other end of the tool to the rod pull it out and measure the distance between and you got the jamb data, deduct 0.06-0.10". and your ready to fly!
Caught it on sale at midwayusa.com for 12.95$ I think they are 14.95$ regular I got the Stoney Point(now Hornady) OAL they cost more and the Frankford is eaiser to use.
You can do the same thing with a wooden dowel rod and a razor blade run it down the bbl. with the bolt closed make a mark with the razor ,( it's thin enough to give the accuracy a pencil or felt tip marker is to thick to get right down to the muzzle crown), then hold the bullet you want to use against the lands , run the rod down again, make the second mark with the razor blade, then use you caliper to measure the distance. I would use a dowel close to the bore size and don't press to hard or you'll flex the dowell and get a bogus reading. It works and it's cheap!