Cbto

[live2huntmt]

I feel really dumb because I have to be missing something very basic here...so can someone please explain this to me in simple terms:

While using a hornady bullet comparator to measure maximum "Cartridge Base To Ogive" or CBTO .... why does it vary with different style bullets?

Shouldn't the measurement from wherever the bullet touches the lands, to the back of the cartridge....be the same for all bullets? I understand the fact that the Cartridge Over All Length or COAL should change because the "nose" of different style bullets is longer/shorter. But the datum line that the comparator touches the bullet at...to the cartridge base seems like it should always be the same.

The only thing I can come up with is that the diameter of the 30 caliber Hornady comparator is not the same diameter of my barrel lands.....therefore the datum line of the comparator is not the datum line of the lands.

Any help is greatly appreciated! Thanks

 

[MagnumManiac]

The reason is because you are NOT measuring a datum on the comparator, but the point at which the ogive is touching the rifling, ogives are different lengths and shapes, there are basically 2 common types used. Secant and Tangent. Then there's hybrids based upon these, which I won't go into.
ie, a partition in bigger cals has a semi spitzer shape, or less pointy, it will contact the rifling sooner than a true spitzer like an Accubond, even if the same weight.
Hope this helps you understand the differences between bullet shapes, and why it is important to actually measure with each type of bullet where it touches the rifling, it isn't uncommon for 2 weights in the same calibre to have the same ogive length, but you won't know, unless you measure it.

Cheers.
gun)

 

[live2huntmt]

Thank You for your response...but I still don't understand. I believe what you are saying to be true, because I have verified it in every rifle I own! However, I cannot wrap my simple brain around how it is physically possible.

The length from the start of my rifling back to my bolt face is a constant.....so why is the measurement from where the bullet contacts the rifling back to the cartridge base NOT?

 

[FEENIX]

Thank You for your response...but I still don't understand. I believe what you are saying to be true, because I have verified it in every rifle I own! However, I cannot wrap my simple brain around how it is physically possible.

The length from the start of my rifling back to my bolt face is a constant.....so why is the measurement from where the bullet contacts the rifling back to the cartridge base NOT?

Try ...

Effects of COAL and CBTO | Part 1 | Berger Bullets Blog

Effects of COAL and CBTO | Part 2 | Berger Bullets Blog

Good luck!

Ed

 

[live2huntmt]

So is it safe to assume that if my bullet comparator diameter was the exact same as my bore diameter....I would have the same CBTO measurement for all bullets?

The variance in CBTO measurements for different style bullets is because the datum line used by my comparator is not the same datum line where the projectile touches the rifling?

 

[Canadian Bushman]

So is it safe to assume that if my bullet comparator diameter was the exact same as my bore diameter....I would have the same CBTO measurement for all bullets?

The variance in CBTO measurements for different style bullets is because the datum line used by my comparator is not the same datum line where the projectile touches the rifling?

CBTO will vary across bullet types because of the different ogives used by different manufacturers, as well as bearing surface and boat tail lengths.

An extreme example would be to compare a wad cutter to a round nose. The wad cutter has a CBTO and a COAL that are the same because the wadcutter is a full diameter at the nose. Now a round nose wont be at the full diameter until the ogive meets the bearing surface.

As ogives change shape the point that contacts the rifling will shift forward or back for a cartridge with a constant COAL.

Now to take things a step further.
The lead angle ( the angle cut on the start of the lands ) will affect how early the bullet contacts the lands.

For a bullet with a standard ogive, a steep lead angle will contact the bullet sooner than a shallow lead angle.

If you are still having trouble grasping the concept just say so and i will draw pictures.

 

[tailbon3]

Correct me if I am wrong here guys because I 'think' I understand this topic but I maybe don't.

Your Cartridge-Over-All-Length with vary with different bullet styles if you seat your bullets to the same CTBO. For example the Barnes TTSX has a long skinny point. Berger VLDs do also. Nosler Partitions have a stubbier nose than those other two. So, if your CBTO is the same for all 3 bullets, the COAL will vary.

Why does this matter? It matters alot if you use a box magazine that doesn't give you much extra length for long bullets. Your Barnes or Bergers may not even fit in your magazine.

Perhaps it matters not at all if you are single loading cartridges directly into your chamber.?.

 

[Canadian Bushman]

Correct me if I am wrong here guys because I 'think' I understand this topic but I maybe don't.

Your Cartridge-Over-All-Length with vary with different bullet styles if you seat your bullets to the same CTBO. For example the Barnes TTSX has a long skinny point. Berger VLDs do also. Nosler Partitions have a stubbier nose than those other two. So, if your CBTO is the same for all 3 bullets, the COAL will vary.

Why does this matter? It matters alot if you use a box magazine that doesn't give you much extra length for long bullets. Your Barnes or Bergers may not even fit in your magazine.

Perhaps it matters not at all if you are single loading cartridges directly into your chamber.?.

We are actually discussing the inverse of what you describe. Which matters if you are trying to find the jump or jam of a particular round loaded to SAAMI spec COAL.

In this particular case we are trying to help the OP understand the cause of CBTO variances he is finding.