What is ogive?

[RockyMtnMT]

And ogive is the style or form of the bullet nose. Not a specific spot on the bullet.

 

[MNbogboy]

And ogive is the style or form of the bullet nose. Not a specific spot on the bullet.

Correct, but acronyms CBTO and BTO are referencing a certain spot (relative) on that bullet ogive.
With machined bullets as yours COAL would probably work in most cases providing ogive land contact distance remained within seating node windows. Some seating node windows however often may fall in plus or minus .003 for optimum grouping. Meplat uniforming on swedged bullets and using COAL may leave something on the table unless real tight bullet sorting tolerances are used.
My humble opinion.

 

[Mikecr]

Other issues here are the common fallacies in purpose.
Bullet matching should boil down to BC, as there is rarely anything in build variance affecting internal ballistics. That is, an actual full blown flaw.

Base to ogive(Bullet BTO) includes base angle, end diameter, base length, bearing length, and bearing to ogive datum. All meaning relatively less to BC, and without considering each individually, you cannot determine an actual mismatch in BC.
If base angle causes smaller end diameter and longer base length while bearing is a tad longer and ogive radius is a bit lower, what happened to BC? Did all combining cause your BTO measure to read higher, lower, or same?

On meplats, the worse thing you can do is trim to make bullets the same OAL. This would leave big variance in meplat diameters, which is big to BC.
To get meplat diameters the same through trimming means trimming from a consistent ogive datum, and now you go back to qualifying ogive radius to get that. Right now, a Bob Green Comparator(BGC) is needed to do it.
And as hard as it seems to measure from a datum high on ogives, try measuring meplat diameters..

You're gonna need a $Keyence laser mic$ to do this stuff with speed & accuracy.
To match BC, you need software to calculate drag per attribute as fed from said laser mic, summed into overall BC (as calculated). With that you have information to credibly act on.
This should be done by QC at the bullet making source, and it might even be viable.
A bullet maker could actually match BC in lots, offering these premium lots, saving everybody else from engaging in the silly pooch poke that BTO measure represents.
It could be automated, bullets go in one chute and come out 10 chutes for ~$250K.

 

[FEENIX]

For what it is worth ...



https://bergerbullets.com/vld-making-shoot/

https://bergerbullets.com/wp-content/uploads/2013/03/COAL.pdf

 

[RockyMtnMT]

Another thing that I am not sure many shooters/loaders understand is how small these measurements of difference are. We hold 2 ten thousands tolerance. Custom barrels are about 5 ten thousands tolerance. Factory barrels, I'm not sure.

I just pulled one of my few remaining hairs and measured it. .0011"! That is so far out of tolerance that I would have s fit if bullets show a variance that big!

 

[RockyMtnMT]

What do you think the chances are that the hole in the latest wiz bang base to ogive bullet measuring devise has a perfectly round hole in it? What happens when the .0002" out of round hole lines up with the .0002" out of round bullet?

 

[Edd]

Regardless of all the specific details, people are still going to measure CBTO length. One of the things that will change that length is the diameter of the contact point on the bullet.

On the bullets I've measured, the distance of a .0002" change in diameter is greater from .2640" to .2638' than it is from .2560" to .2558".

 

[RockyMtnMT]

My point is that the cbto tool is measuring dia on an arbitrary point on the ogive. A diff of .0002" in dia from one bullet to another will show significantly larger change in length than .0002" . it is a false indicator.

 

[Mikecr]

On the bullets I've measured, the distance of a .0002" change in diameter is greater from .2640" to .2638' than it is from .2560" to .2558".

This is likely due to difference in ogive radius between a 26cal & 25cal bullet. If their ogive radius was the same then cal diameter would not cause the error to change as you describe. And if secant ogives, the error is way larger than tangent.

On another note, ogive radius also cannot be measured with one datum. It takes at least two different points on the same nose to determine ogive radius. And with hybrids these points must be each on the different forms (one on tangent section, the other on secant section, neither on transition).

Ogive radius variance is a real factor with mass manufactured bullets. It can significantly change where your tool takes datum. It takes a bit of thought to understand it, but in reality this does not affect land relationship with seated bullets like you might think it would. It's self compensating in that regard.
However, when trying to match bullets, it's a problem.
With this, don't even attempt to match bullets until qualifying ogives from two points of measure. This is what the Bob Green Tool(BGC) does at once, but some folks measure two points separately. I've also done it separately with a blade mic/indicator rig, and washed numbers through software to get actual ogive radius per nose instead of merely comparing noses. An educational endeavor that IMO is not viable to hunting or even competition.