OAL guage
Insert your losded round in the proper cal. hole, measure with calipers from cose head to other side of tool. Then measure the tool only across the opposing flats and subtract that from your shell reading. That gives you the OAL of that round. If you need to make it longer or shorter, you need to know how far it is from bolt face to rifling. Or you can check a factory round that works well in your rifle and duplicate that OAL. Al