Well, I'm not quite sure if my last post was quite at the proverbial thousand words mark, but it was close...

This is what I was trying to say:



If you start with a zeroed caliper, then install the bullet comparator, THEN measure the loaded cartridge, you'll get the total length of the cartridge plus the comparator body, minus the tip of the bullet. This is "X". The calibration length for your comparator (the 1.007" you referenced before) is Y. Then X-Y = Z, which is the length from your bolt face to the ogive of your bullet.

Below is how I used to use the hornady comparator and OAL spindle. I've since gone back to using magic marker to 'smoke' my bullets again.

This is what I consider the PROPER WAY to determine seating depth and COAL using a bullet comparator:

This technique is based on a establishing and maintain a desired bullet jump for a given rifle's chamber. I want to hold this bullet jump constant no matter what bullet I use, or if the tip of a bullet is damaged. There's more than one "right way," but this is how I've used it with good results.

The diagram below is an exaggeration comparing the ogive positions and proper COAL/Seating Depth determinations for a long profile spire point and a heavy round nosed bullet. This compares two methods for seating bullets based on two different control lengths. The WRONG path A-->B-->E uses a fixed COAL, but as you can see, it ends up seating the bullet too long and jamming the rifling - which can be dangerous. The CORRECT PATH: A-->D-->F-->C determines the proper seating depth for a given bullet, then determines the proper Ogive position for THAT RIFLE and keeps the OGIVE POSITION CONSTANT, meaning the COAL would change between different bullets.




So this is a bit exaggerated, but this is what I'm talking about.

If a reloader only goes by COAL targets, they can get themselves in trouble. Using a bullet comparator, IN THEORY, will let you properly seat any bullet to match your rifles chamber:



In this image, Cartridges A, B, and C from the caliper graphic are shown. A was properly seated based on smoke testing or Hornady OAL spindle. B was NOT properly seated, because the reloader used the same COAL as A, but with a fatter profile bullet so it's now jammed heavily into the rifling, which may create a dangerous pressure condition. The bullet ogive for Cartridge C was matched to A by using a comparator, so the proper bullet jump was maintained, and the bullet was properly seated to match the rifle in use.

The process:

Determine the proper seating depth for your specific bullet to give the bullet jump that you want - doesn't matter whether you use the Hornady OAL spindle and Modified Case or old fashioned "smoking" technique. This produces the cartridge in Caliper A.

Once you establish your exact COAL appropriate for THAT BULLET IN THAT RIFLE based on its ogive position, you can use the bullet comparator to determine the distance between your bolt face and ogive position to produce your desired bullet jump for THAT RIFLE, that will be consistent for ANY BULLET. Caliper D above.

Then if you ever choose to change bullets, you can seat the new bullet to have the same bullet jump by seating the bullet ogive to the exact same position with the comparator. Caliper F above.

I personally just set my dies based on the comparator, but if your tips aren't deformed, you can then use that properly seated bullet to determine the appropriate COAL for THAT RIFLE with THAT BULLET. Caliper C above.

OR YOU COULD JUST SMOKE THE NEW BULLET THE SAME WAY YOU DID THE ORIGINAL BULLET.

The WRONG PATH: Some guys will fixate on a given COAL, but COAL is 100% irrelevant as you change bullets because the profile changes. Keeping COAL the same but changing bullet profiles would change your ogive position relative to your bolt face, meaning your bullet jump changes, which could create a dangerous pressure condition.

You've determined the proper seating depth and COAL to give you your desired bullet jump with the first bullet. Caliper A.

Then you use the SAME COAL for your new bullet. Caliper B.

But the heavy round nosed bullet obviously has it's ogive further forward than the spire point. When measuring this bullet seated to the same COAL as the original bullet, it's now jamming the rifling, which might over create an over-pressure condition. This is indicated in Caliper E above, the length from the case base to the bullet ogive is much longer.