Clarification needed on variations in OAL to ogive

[KrazyKowboy]

Hello all,

I ran a couple searches but did not necessarily find the clarification I was looking for. For years I have been using the method of inserting an overlength cartridge into the chamber (stripped bolt) to the rifling and measuring from the muzzle and then taking measurement with he bolt closed. Taking the difference and subtracting the ogive to meplate measurement. (Hope that makes sense). I always redo this process for each weight and style of projectile.

Recently I purchased a OAL gauge and although this will speed up the process and possibly be more precise I am forced to mentally revisit a concept that has nagged at the back of my mind since I began doing the old school method:

Why does the CBTO vary from bullet to bullet?

Obviously, the actual distance from bolt face to the throat cannot change (discounting for throat burnout). So, given that the chamber length is constant, the variation has to be in the bullets and ogive position and size. But a change in position would not cause a change in CBTO because the cartridge is against the bolt face and the same diameter ogive would engage in the same spot. So the difference should be ogive size but that wouldn't really work with consistent(relatively speaking) bullet diameters.

All that said, the only thing I can think of that could still cause this discrepancy would be if the "comparator's ogive" point of measurement was not the same as the "rifle's ogive" point of measurement(aka where the bullet seats into rifling).

Is that where the discrepancy is? Is it something else entirely I've missed? Please help me understand. Thank you for your time in advance.

-T

 

[gconnoyer]

Why does the OAL(to ogive) vary from bullet to bullet?
-machining/manufacturing tolerances

The comparator should measure off of the Ogive of the bullet if you have the correct insert.
The variance in Ogive location is from a longer/shorter base to ogive based on the manufacturing of that lot.

So depending on where your seating die seats the bullet from, your measurement off the lands should stay the same.

Measuring from the muzzle and off of the tip of the bullet and doing math is a long way of doing things, and with bullet variance (especially measuring off of the bullet tip) is going to give you results all over the place.

 

[KrazyKowboy]

Why does the OAL(to ogive) vary from bullet to bullet?
-machining/manufacturing tolerances

The comparator should measure off of the Ogive of the bullet if you have the correct insert.
The variance in Ogive location is from a longer/shorter base to ogive based on the manufacturing of that lot.

So depending on where your seating die seats the bullet from, your measurement off the lands should stay the same.

Measuring from the muzzle and off of the tip of the bullet and doing math is a long way of doing things, and with bullet variance (especially measuring off of the bullet tip) is going to give you results all over the place.

Thanks for your response. I had actually watched that video at one point and have done something similar it just seemed subjective to me I guess maybe I will revise it. To clarify, the actual measurement I have used in the past was from cartridge base to ogive not tip, but I agree it is not the most consistent method. To rephrase my question: why do I have to remeasure OAL with every new bullet style/weight? Why can't I just set the length to ogive back my desired setback and go? Thanks again for taking the time to answer.

 

[spife7980]

Thats a cluster f of a post to try and decipher what the hell youre even trying to ask.

CBTO- Case Base To Ogive
COAL/COL- Cartridge Over All Length/Cartridge Overall Length

Saying COAL and meaning CBTO is wrong, plain and simple. Communicate what you mean properly and it will help keep miscommunication and misinformation minimized.

And the ogive diameters and locations arent always the same, they vary lot to lot and depending on the quality of the bullet even within the same lot.
And youre not measuring between two parallel surfaces, you are measuring to the lands which are at an angle, between 1-3 degrees usually. Add onto that fact that the comparator you are using to measure isnt contacting in the same exact spot and angle as your chamber.


So the differences you are seeing are due to the bullet, the chamber, the tools to measure it. All of it.
http://bergerbullets.com/effects-of...coal-and-cartridge-base-to-ogive-cbto-part-2/

 

[Sheldon N]

Thanks for your response. I had actually watched that video at one point and have done something similar it just seemed subjective to me I guess maybe I will revise it. To clarify, the actual measurement I have used in the past was from cartridge base to ogive not tip, but I agree it is not the most consistent method. To rephrase my question: why do I have to remeasure OAL with every new bullet style/weight? Why can't I just set the length to ogive back my desired setback and go? Thanks again for taking the time to answer.

That method is not subjectve at all, you can very easily feel right down to 0.001 where the bullet no longer encounters the lands.

Regarding your second question, the reason you see difference between bullet types is because the ogive tool is not actually measuring the ogive. It's measuring slightly forward on the bullet from where the lands would actually touch. Each bullet type has a different nose shape, which means a different angle, which means a different measurement. Another way to phrase it might be that when you measure BTO you are actually measuring "base to comparator contact point".

 

[gconnoyer]

To rephrase my question: why do I have to remeasure OAL with every new bullet style/weight? Why can't I just set the length to ogive back my desired setback and go?

Ideally your die setup and bullet choice should give you the same OAL from case base to ogive each time, so your "off the lands" measurement should be consistent. You'll never know measuring your method with the tip through the muzzle though.

The difference in the bullet itself will most likely stick the base of the bullet deeper into the case.

How much difference that actually makes, I can't say.
I would think it would change case pressure, maybe velocity, it may not be a big enough amount to worry about. But I order enough bullets to burn a barrel, or measure the new lot of bullets and do a new load development or atleast check my current load.

 

[lash]

Okay, based upon your clarification in your second post, your best informational answers are in @spife7980 and @Sheldon N ’s posts.

Differences in bullet designs vary considerably, causing the actual ogive to be in very different places on a bullet A to bullet B comparison. Add to that the facts that not only are your bullet comparitor inserts usually not locating at exactly the ogive in a given bullet (per the article linked by Spife), but your bullet seating die is not seating off of the actual ogive.

An example of this is to compare the bullet shapes and dimensions of the following two 6.5 mm bullets; Sierra match king 142g and the Berger hybrid 140g. Compare the shapes of the nose ahead of each bullet’s ogive to the lead-in angle of the rifling and you’ll see that each of these will engage the rifling differently.

 

[KrazyKowboy]

Thats a cluster f of a post to try and decipher what the hell youre even trying to ask.

CBTO- Case Base To Ogive
COAL/COL- Cartridge Over All Length/Cartridge Overall Length

Saying COAL and meaning CBTO is wrong, plain and simple. Communicate what you mean properly and it will help keep miscommunication and misinformation minimized.

And the ogive diameters and locations arent always the same, they vary lot to lot and depending on the quality of the bullet even within the same lot.
And youre not measuring between two parallel surfaces, you are measuring to the lands which are at an angle, between 1-3 degrees usually. Add onto that fact that the comparator you are using to measure isnt contacting in the same exact spot and angle as your chamber.


So the differences you are seeing are due to the bullet, the chamber, the tools to measure it. All of it.
http://bergerbullets.com/effects-of...coal-and-cartridge-base-to-ogive-cbto-part-2/

LOL thanks I did change a couple of terms to help clarify so thank you for that because I did not know that was the proper term instead of writing out a confusing sentence. Thanks. The rest of your answer was spot on so thank you, that helped me understand the discrepancy between various bullets.

 

[KrazyKowboy]

Thanks folks! I think I got it now, and a better way to measure distance to lands. Appreciate the help all!

 

[Desert Ranger Tycho]

Thats a cluster f of a post to try and decipher what the hell youre even trying to ask.

CBTO- Case Base To Ogive
COAL/COL- Cartridge Over All Length/Cartridge Overall Length

Saying COAL and meaning CBTO is wrong, plain and simple. Communicate what you mean properly and it will help keep miscommunication and misinformation minimized.

And the ogive diameters and locations arent always the same, they vary lot to lot and depending on the quality of the bullet even within the same lot.
And youre not measuring between two parallel surfaces, you are measuring to the lands which are at an angle, between 1-3 degrees usually. Add onto that fact that the comparator you are using to measure isnt contacting in the same exact spot and angle as your chamber.


So the differences you are seeing are due to the bullet, the chamber, the tools to measure it. All of it.
http://bergerbullets.com/effects-of...coal-and-cartridge-base-to-ogive-cbto-part-2/

Mahalo for sharing your knowledge on this topic. I am fairly new to the reloading game. I will summarize my response to the info on this thread:

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