variation in loaded round CBTO

[Chaotik]

I was loading 25 rounds for a trip to the range (100yard) tomorrow to sight in a new scope and start a load workup. Caliber is 6 dasher and I was using 1x fired and annealed 6br Lapua brass (fireforming using the false shoulder method). Berger Hybrid 105 bullets were initially sorted into lots using a BGC, and then I picked a lot and sorted this lot further by OAL. I did not sort bullets bullets by BTO.

As I was seating the bullets (using 21st Century hydro press) I notice that a couple of the round were about .010 longer than the remainder (all were to be seated to touch the rifling). There was no significant difference in seating force (all < 10psi on the press/necks were lubed) using a Wilson micrometer seating die (custom cut with reamer for chamber). Measurements of CBTO were made with a Short Action Custom comparator and a mitutoya caliber.

Not sure what could explain this variation in loaded round CBTO.

 

[6bra1k]

in the real world the is no requirement for the tip length and the ogive length to relate to each other, it is why we do NOT measure tip to base on competition ammo..( br) if this is to be mag fed...well you may have an issue but i doubt it

 

[RegionRat]

Things to watch for...
Complete bullet inspection to know ahead of seating them when their overall length variation is different than their ogive variation.
Compression loads can change seating after the fact.
Seating forces can vary, and when they are small to being with may allow change after the fact with compression loads.

 

[mikecr]

Sounds to me like you have everything covered.
I wonder if the BGC is right in base form to work with hybrid ogives.

You transition between (2) different ogive types.
So one datum of your BGC rests on the tangent portion of ogive, while it's second datum rests on what amounts to another/secant ogive. So you're not comparing ogive radius, but comparing datums across two different ogives.
It's not the same.

For a hybrid, the important portion is the tangent section, so maybe you need one cal higher of seater plug in your BGC, to compare purely tangent portion radius.
You could adapt a 25, 26, or 27cal Wilson seater plug (for your 24cal BGC) to do this.

 

[rwj]

Not sure what could explain this variation in loaded round CBTO.

The seating stem contacts the bullet at a different diameter than the gauge…

 

[Chaotik]

All of my F class and benchrest rifles are single shot, so loading in a magazine is not an issue.

The only way, in my mind, to explain this discrepancy, would have to be a variation in the distance between where the seating stem makes contact on the bullet nose and the ogive of the bullet.

As I have understood it, this is what is measured using the Bob Green Comparator, which made me go back and look at this.

I believe that my error was in misreading the BGC by a full 0.0100”. The indicator is an analog dial and a complete 360° rotation of the needle represents 0.010”. I was only reading the last 2 digits (the one thousand and one ten thousand places) and failed to realize that the needle had spun completely around an additional 0.010” when I inserted the bullet to get the measurement.

 

[JMayo]

Jacket difference, meplat difference.


I do sort by oal so do several other competition shooters I know.

You Get the true coefficient that way. I don't want the shortest one next to the longest one in a string.

 

[mikecr]

The seating stem contacts the bullet at a different diameter than the gauge…

He measures CBTO from gauge datum, not from seating stem datum.
The BGC qualifies ogive radius (shape) with each nose, so that any measure off a nose is qualified.
OP's problem is different for sure, and hopefully was a mere misread of his dial.

 

[rwj]

He measures CBTO from gauge datum, not from seating stem datum.
The BGC qualifies ogive radius (shape) with each nose, so that any measure off a nose is qualified.
OP's problem is different for sure, and hopefully was a mere misread of his dial.

Are the gauge and seating stem datums the same?

 

[K22]

Another example to illustrate how the published max COL (measuring to tip) can be misleading is with the 6mm, Sierra 85 BTHP bullet.

Because of the ogive nature of this bullet, if I was to seat it using the published max COL (i.e., 2.710) or even that listed in the Sierra Manual test load for this bullet (i.e., 2.650) the cartridge would not chamber in my 243 Tikka or Browning X Bolt but does chamber in the Rem 700.

This is why it's important to always measure the true maximum COL for any bullet / rifle combination. I use a version of the Frankfurt Arsenal tool which bases max COL on the bullet ogive, not the bullet tip.

Also, for most bullets, even high quality one's like Sierra, it's been my experience that bullet ogives will vary a few thousandths even in the same lot. Since for my purposes, I want to avoid jamming the bullet into the lands, I back of the seating depth to no closer than .010" from the lands to account for this variation.

 

[mikecr]

They're different animals. But qualified ogives provide the different datums at the same distance apart.
If seating forces are the same using qualified ogives, CBTOs will match.
OP's seemingly didn't match. That was a weird thing

 

[rwj]

They're different animals. But qualified ogives provide the different datums at the same distance apart.
If seating forces are the same using qualified ogives, CBTOs will match.
OP's seemingly didn't match. That was a weird thing

What is a qualified ogive?

 

[dgeesaman]

I was loading 25 rounds for a trip to the range (100yard) tomorrow to sight in a new scope and start a load workup. Caliber is 6 dasher and I was using 1x fired and annealed 6br Lapua brass (fireforming using the false shoulder method). Berger Hybrid 105 bullets were initially sorted into lots using a BGC, and then I picked a lot and sorted this lot further by OAL. I did not sort bullets bullets by BTO.

As I was seating the bullets (using 21st Century hydro press) I notice that a couple of the round were about .010 longer than the remainder (all were to be seated to touch the rifling). There was no significant difference in seating force (all < 10psi on the press/necks were lubed) using a Wilson micrometer seating die (custom cut with reamer for chamber). Measurements of CBTO were made with a Short Action Custom comparator and a mitutoya caliber.

Not sure what could explain this variation in loaded round CBTO.

The Bob Green comparator could be useful here to confirm whether this is coming from variances in the bullets shape near the ogive.

The reason this matters is because the seater stem, comparator, and rifle lands all touch on a different “ogive” diameter and if the bullet shape isn’t dead consistent, it will be just as you’re experiencing now.

ETA I should have read further, you’re familiar with this.

 

[Coyotefurharvester]

And he solved his own question, realizing he misread the dial caliper.

 

[wkdickinson]

The seating stem contacts the bullet at a different diameter than the gauge…

You are seeing variations in the form of the ogive. I have had the same issue with Berger 105 hybrids. I made an insert for my compariitor that is approximately the same diameter as the seating stem in my Whidden die. Then I compared the measurement between that and the standard insert (Hornaday) and for the lot of bullets I have there is significant variation.

 

[Ned Ludd]

All of my F class and benchrest rifles are single shot, so loading in a magazine is not an issue.

The only way, in my mind, to explain this discrepancy, would have to be a variation in the distance between where the seating stem makes contact on the bullet nose and the ogive of the bullet.

As I have understood it, this is what is measured using the Bob Green Comparator, which made me go back and look at this.

I believe that my error was in misreading the BGC by a full 0.0100”. The indicator is an analog dial and a complete 360° rotation of the needle represents 0.010”. I was only reading the last 2 digits (the one thousand and one ten thousand places) and failed to realize that the needle had spun completely around an additional 0.010” when I inserted the bullet to get the measurement.

This is what I was thinking when I first read your post, whether it could be mis-reading the tool by .010". That is an enormous difference in CBTO when the bullets have been sorted as you described. Once you have sorted bullets using the BGC and by OAL, you should not have .010" variance in CBTO. Making CBTO measurements more uniform is the main reason for doing such sorting techniques. The only other thing I could think of would be a press malfunction, which didn't seem likely in light of the other information. It's always a good feeling when you figure out the issue on your own.

 

[mikecr]

What is a qualified ogive?

In my context, qualified is meeting a standard. In this case, a shape standard.
With all ogives meeting a standard shape, then datums taken off them are qualified, and not until then.

There are a number of things in reloading that are commonly not qualified(taken to standard) in any way.
Things like primer seating, primer striking, shoulder bumping, dynamic capacity, and neck tension among them.

 

[PJ1]

I thought this was a rather good thread on this topic:

consistent base to ogive measurement

Seeking more consistent base to ogive measurements - is it the die or my own technique? Currently using .308 Forster BR seater die, have replaced the spring and the seating stem. Out of a typical batch of 25 reloads the base to ogive variance can be 0.003" with 155 gr Sierra 2156's...

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