Your C.O.L. will change about every 200 rounds down the pipe on a 243. I use a Stoney Point gauge and Sinclair comparator and all that. But you can measure a bullet dropped in the bore with jerry rigged rods. I used to just seat a bullet out long. Chamber it. Then measure the rifling scratch with calipers and a magnify glass. Whatever the scratch length is will be a rough + contact datum point. For a while.