I consider COAL and OAL the same thing and is defined on the SAAMI Glossary
SAAMI | Product Manuals
in the 2nd definition
1. Firearm: The dimension measured parallel to the axis of the bore from the muzzle to a line at right angles to the axis and tangent to the rearmost point of the butt-plate or grip.
2. Ammunition: The greatest dimension of a loaded cartridge, i.e., from face of the head to the tip of the bullet for centerfire or rimfire or to the crimp for shotshells or blanks (not to be confused with the uncrimped length in a shotshell.) "
You are not measuring the COAL or the OAL with the comparator. You are measuring to the point on the ogive that is close to caliber dimension. That is shorter than COAL but much more useful.