CBTO

[RockyMtnMT]

This has been discussed in the past. I just want to see what the current membership thinks. CBTO stands for Case Base To Ogive as a measurement having to do with cartridge length. What is an Ogive? We can debate pronunciation too. That's always fun too. No cheating and looking back a few years for past discussions!

 

[lancetkenyon]

I say oh-jive. But I have heard it pronounce ah-give, oh-give, and even ah-jive.
Ogive is actually the entire curved portion of the bullet in front of the bearing surface.
But, when saying you are measuring CBTO, it is usually referring to the distance from the case base (head of case) to the spot on the bullet where the ogive meets the bearing surface.

There are some variations between comparator inserts, so it might not be exactly at that point, as the comparator is actually slightly undersized from the bullet diameter, or else it would slide over the entire bullet. Like a 6.5mm comparator insert is actually probably .262"-.263".

 

[DoneNOut]

Welp, Lance nailed it. No need for further discussion...

 

[lancetkenyon]

Should I have prefaced it with "Spoiler alert"?

 

[RockyMtnMT]

So, does CBTO tell one where the bullet will touch the lands? Yes the ogive is the entire portion of the bullet nose from the meplat to the shoulder. It is not a single point on the bullet. This is the misconception of the definition of ogive.

 

[Gregg C]

Isn't CBTO more or less just a reference, a more accurate, repeatable reference, to cartridge length? This is how I understand it.

 

[Koda_]

So, does CBTO tell one where the bullet will touch the lands?

No. But it is a more accurate way to measure consistent seating depth.. and thus consistent jump to lands.

For many bullets the tip is not a reliable place to measure from.

 

[elkaholic]

So, does CBTO tell one where the bullet will touch the lands? Yes the ogive is the entire portion of the bullet nose from the meplat to the shoulder. It is not a single point on the bullet. This is the misconception of the definition of ogive.

its from the base to where the ogive starts. The distance would be slightly longer before it touched the lands, but not much and depending on secant or tangent as well as degree of angle.

 

[HammerBullets]

Following

 

[Longtine88]

This has been discussed in the past. I just want to see what the current membership thinks. CBTO stands for Case Base To Ogive as a measurement having to do with cartridge length. What is an Ogive? We can debate pronunciation too. That's always fun too. No cheating and looking back a few years for past discussions!

Yeah we all do it to some degree on the ogive of the nose. People think it's like a tangent of a Gothic arch type measurement and it would be but in actuality the lands shorten the diameter of the bullet by around 0.008" total. Example: .264 cal bullet - 0.008"(land height) so the actual tangent width to where everyone is measuring a CBTO measurement is not the true tangent and this measure will change from different brands or types of bullets because of the secant angle from tip of bullet to tangent point of the curvature.

Beer time!!!!

 

[lancetkenyon]

So, does CBTO tell one where the bullet will touch the lands? Yes the ogive is the entire portion of the bullet nose from the meplat to the shoulder. It is not a single point on the bullet. This is the misconception of the definition of ogive.

Not necessarily in my opinion.
FB, leade/throat, bullet shape all make differences in CBTO TO LANDS.

Just because 1 bullet may be 2.188" to lands, doesn't mean all will be.

With a new chamber, look at the differences in CBTO/COAL with different bullets. All due to ogive shape.

 

[cdherman]

So, does CBTO tell one where the bullet will touch the lands? Yes the ogive is the entire portion of the bullet nose from the meplat to the shoulder. It is not a single point on the bullet. This is the misconception of the definition of ogive.

So yes, I hear what you are saying CBTO in what most of us are "meaning" is cartridge base to ogive contacting lands" so to speak CBTOCL;

All of the comparators are estimations. CBTO or cartridge base to ogive will remain my acronym of choice. Taken literally as RockyMtnMt points out, its a useless number since the ogive of a bullet stretches over a long distance. Take to mean cartridge base to where the ogive would hit the lands first is what we are wanting to measure.

As noted, it is not a perfect measurement. And the variation you may get in velocity and pressures seems to get wilder, the closer you get to the lands. That's why some BR people used to (still") jamb a bullet into the lands. Coupled with a lightly sized neck, the bullet was always seated the same into the lands.

However, trying to seat a bullet consistently, say 0.002 off the lands proves nearly impossible. And so seating that close seems to never result in the same accuracy. Thus, I will say that most of use aim to start testing at say 10 though off and go longer.

 

[L.Sherm]

I never start at .010 off, always touch and work back. I dont use the Hornandy style inserts anymore either. Ive switched to the whidden style that uses bushings like if I'm doing it for a 6.5 I use a .264 bushing.

 

[RockyMtnMT]

Here is my take on it now that we have established that the ogive is not a finite point location of a bullet. Sliding something onto the nose or ogive of the bullet hoping that it will stop at a point of consistency better than inconsistent meplats of many bullets has turned into a marketing gimmick that misleads shooters into thinking they can use this info to chamber a rifle to exact specifications. Then marketed as a must have tool for the loading bench that most people have no use for.