Loading manuals often list the COAL for the bullet as the distance from base to tip. That measurement (base to tip) will vary, of course, depending on the bullet you select and the seating depth you choose for that bullet. When you use a gauge (like the Hornady OAL Gauge) to measure the distance to lands, the bullet tip doesn't stop at the end of the free fore; the bullet's nose extends into the lands. The COAL (base to tip combined overall length) you measure by pushing the case/bullet combination until it stops will produce a number greater than the number listed in the manual.
Most reloaders use CBTO (combined base to ogive) as a constant when working on reloading technical data. That's because it is a better indicator of how far the bullet travels before contacting the lands. You would need something like a Hornady OAL Length Gauge to make those comparative measurements.
The term COAL is often used when CBTO is actually what's being described and that can lead to unnecessary confusion.
Hornady OAL Gauge - YouTube
Using the "one caliber seating depth" theory may work in some instances, at least as a starting point, but plugging in such approximations as boat tail lengths etc. are unreliable factors. How long is a "boat tail" and how does it compare mathematically with the caliber? Using the caliber as a seating depth produces no useful test data because bullets of different weights are typically different lengths so seating a 160 grain .30 cal bullet .030 into the case will give you a different COAL than doing the same thing with a 200 grain .30 cal bullet. The Berger 200 grain 30 cal. hybrid is listed as 1.50 in length. The Berger 168 grain 30 cal hybrid is listed at 1.296 in length. Your COAL and distance to lands using these bullets will vary .204 if you used the "one calibe" seating depth. Who want's to rely on those kinds of variables?
