The old timer/owner of a local gun shop told me how he measures this. He would close the bolt and insert a rod (unknown material- wood, metal, plastic) until it touches the bolt face. He would then mark the rod where it exits the barrel. Then he would insert a round that was loaded long David Tubb style and push it into the lands and then re-insert the rod until it touches the point of the bullet and mark the rod. The distance between the marks is the length of cartridge touching the lands. I dont know why you couldn't just measure the cartridge after it has been pushed into the rifleing, maybe it would be pulled back out of the neck when removed. Does anybody else do this?