I'll also reccommend the compartorer. But if you only have one rifle and you don't want to spend the cash of a comparator, you can probably make a workable substitute by drilling a hole thru the primer pocket of a fired case and inserting a rod (or whatever) into it and a bullet into the case mouth. Remove the bolt of your rifle chamber the case and push the bullet until you feel the bullet touch the rifling. Mark the rod (or whatever) even with the head of the case and do the math and measure properly, and you can figure the COAL.
I bought a comparator, but you have to have a separate case for each caliber. I drilled and threaded extra cases and use the rule in the comparator. But I don't see why you couldn't make a reasonable one.
With the comparator or with pushing the bullet with the bolt, as the OR desicribed, the rifling will grip the bullet and displace it, and (especially pushing the bullet with the bolt) you can never be quite sure you're getting a good and accurate COAL measure.