The best way to do it is with the right tool, but this how I did it before I obtained a Stoney Point Overall Cartridge Length (O.A.L.) Gauge. I would pick the most uniform bullet I could find in the box of bullets I was going to load. I would place just the bullet in the chamber and very very very so lightly tap it into the rifling so it will just stay there. I would then set the rifle on it' s butt, and place a wood dowel or cleaning rod down the muzzle contacting the tip of the bullet. I then would place a very thin mark around the dowel buy rotating it. I would then tap the bullet out and reinstall the bolt, reinsert the dowel, and make another thin circular mark around it. I then removed the dowel and measured the differnece between the two lines with a dial, or eletronic digital caliper and that was my O.A.L. It is not as accurate as using a Stoney Point O.A.L. Gauge but it is pretty close. Good luck.