It is now clear to me that bullet taper plays a big part to COAL.
Glad you recognize that. Except it doesn't, or at least not all by itself.
datum point
Any reference point of known or assumed coordinates from which calculation or measurements may be taken.
yep, that's the definition. ANY POINT you choose. SO most of the confusion comes when I pick one point and you pick another..or so it seems to me.
And there are some other things to consider. COAL (Cartridge Over All Length -aka overall loaded length with bullet) is measured from the base of the case to the tip of the bullet. Not to any "datum point" on the ogive but to the tip of the bullet. That's the datum point for COAL.
And COAL as listed in the loading manuals is the maximum length by SAAMI standards. However, all that means in reality is its the max length by SAAMI standards.
Sound confusing?? it can be..
Understand that the SAAMI standards for COAL are not "you must meet this or DIE!!!" kind of things. They are agreed upon industry standards to produce ammo that is expected to work in all firearms chambered for that cartridge.
Some guns will work with rounds that are slightly longer, some will not, and some need rounds that are slightly shorter than the max standard length.
AND, we're talking about two important measurements here, the length that will chamber and NOT be in contact with the rifling, and the length that will work through the gun's magazine and feed into the chamber.
Generally they are NOT the same. The SAAMI length is intended to be off the rifling AND work through the magazine. And, to further muddy the water, there are guns where one can load to just off the lands, and be too long to work through the magazine. AND, it can change when you change bullets.
A good illustration of this is the difference in COAL between a spitzer seated to X.xxx" length and a flat point/round nose bullet of the same weight, seated in the same die with no changes to adjustment.