I borrowed this picture from UncleNick and posted it on another forum. The question was asked, "what OAL for a 45 ACP LSWC", so I put up this picture.
I got spanked by the local reloading guru and I wasn't sure how to reply.
Here is some of what he said about this method of determining OAL.
Using the barrel as a gage is OK but do not allow the cartridge to headspace on the bullet. That is NOT OK! The pictures posted by steve4102 indicate it is OK (headspace on the bullet, third choice from the left) and that does not agree with major reloading companies reloading manuals.
In no case should you ever allow the cartridge to headspace on the bullet. That is not OK. The bullet needs some run on engaging the rifling. With out that run the bullet stalls in its exit from the case and the fast rising pressures can become too high. While it may not blow up the gun it may do a number on the case, or blow the primer out of the case and into your face (hot primers on the skin are not conducive to smooth skin!).