Comparative locations of ogive concerning COAL

[Tiny Tim]

I am reloading for a Ruger m77 IN 7mm RM with both Berger 168 gr VLD and Standard 160gr Nosler AB. I have been reloading for 25+ years. Since my rifle has so much free bore, I've been limited by magazine length. In the process of trying to shoot more accurately, I'm trying to determine my COAL so bullets are just touching the lands. After determining this length for each bullet I found them to be only .020" different (Berger being longer). Then when measuring from the base of the case to the ogive of the bullets, I found this dimension to be approx .090" longer on the AB. I would have assumed that this dimension (cartridge base to ogive) would have been very similar for ANY bullet and the COAL to the tip varying widely as determined by bullet profile. Is this an optical illusion making the Berger appear as if it would have a significantly longer COAL due to its profile? Is my reasoning wrong? Did I miss something? Sorry this post is so long. Any help understanding this would be greatly appreciated

 

[FearNoWind]

Sure enough Tim, the ogive radius differs from one bullet design to the next.
There is no way to compare COAL with CBTO.

 

[Mikecr]

Since my rifle has so much free bore, I've been limited by magazine length.

Not true,, makes no sense.

In the process of trying to shoot more accurately, I'm trying to determine my COAL so bullets are just touching the lands.

Again, no reason to believe this and makes no sense at all.

You should focus on testing for most accurate bullet & seating depth, whatever that turns out to be. And then log CBTO with that best bullet & seating so that you will reproduce it for the life of the barrel.

 

[TOM H]

I am reloading for a Ruger m77 IN 7mm RM with both Berger 168 gr VLD and Standard 160gr Nosler AB. I have been reloading for 25+ years. Since my rifle has so much free bore, I've been limited by magazine length. In the process of trying to shoot more accurately, I'm trying to determine my COAL so bullets are just touching the lands. After determining this length for each bullet I found them to be only .020" different (Berger being longer). Then when measuring from the base of the case to the ogive of the bullets, I found this dimension to be approx .090" longer on the AB. I would have assumed that this dimension (cartridge base to ogive) would have been very similar for ANY bullet and the COAL to the tip varying widely as determined by bullet profile. Is this an optical illusion making the Berger appear as if it would have a significantly longer COAL due to its profile? Is my reasoning wrong? Did I miss something? Sorry this post is so long. Any help understanding this would be greatly appreciated

You might want to read this article and do click on part 2

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

 

[Tiny Tim]

Not true,, makes no sense.
Again, no reason to believe this and makes no sense at all.

You should focus on testing for most accurate bullet & seating depth, whatever that turns out to be. And then log CBTO with that best bullet & seating so that you will reproduce it for the life of the barrel.

I agree. Makes no sense. I have tried multiple times and get resuls that are within a couple thousandths. I can only assume I am doing something wrong in the process and believe it to be wrong with the Bergers. Am I correct that the cartridge base to Ogive length should always be the same, regardless of bullet brand/profile as it is a gun specific dimension?

 

[tbrice23]

I agree. Makes no sense. I have tried multiple times and get resuls that are within a couple thousandths. I can only assume I am doing something wrong in the process and believe it to be wrong with the Bergers. Am I correct that the cartridge base to Ogive length should always be the same, regardless of bullet brand/profile as it is a gun specific dimension?

No. Hardly ever.

It gets worse, the next box of the same make and weight bullets from a different lot number will be different also.

 

[Tiny Tim]

Thank you. Still trying to make sense of it all. Simply opposite of what I expected.

 

[tbrice23]

It sux, I know.

Your comparator is a few thousandths smaller than the bore of your rifle where the lands meet a certain point of the bullet,

Your lands are at a different angle than the profile of your bullet.

Different bullets have different ogive profiles / curves / slopes.

These points correspond to a different length of where the comparator measures.


Hard to EASILY explain . Hope I'm helping.
Don't forget that this length will change as your throat erodes.

 

[LaHunter]

TT,

1) Your bullet comparator is not contacting the bullet at the same location that the lands of your barrel will contact the bullet. The shape of the bullet greatly impacts the CBTO measurement.
2) You can't use the CBTO measurement as a 'general rule of thumb' to compare cartridges loaded with different bullets for your rifle. This measurement is specific for the ammo loaded with a specific type / weight bullet.
3) Touching the lands is no more or less accurate than any other seating depth. The best seating depth is specific for your rifle, with a specific load. Finding the best seating depth is part of load development.

Hope this helps

 

[Tiny Tim]

Thanks everyone. This helps ALOT! This has simply been the beginning of trying to extract the most potential from my gun and looking for a baseline to build from. Sounds like if I find something I like, I best buy in bulk. Thanks again

 

[Corey Schwanz]

Always need to stick to 1 comparator and 1 bullet at a time. Make a dummy round so that the bullet is just touching the lands (lots of tools to help you with this if needed). Then measure that dummy round with whatever comparator you have and note the length. This is the CBTO measurement for that bullet in that rifle with that comparator. You now have a baseline measurement to work from and can experiment with whatever distance of jump you want to test until you find the jump distance that gives you the best accuracy.

When you switch to a different bullet, you need to re-establish your baseline "touching the lands" measurement.

As previously said, different bullets have different shapes so the comparators will contact them at different points on the nose. Also, not all comparators are cut the same so you can get different measurements of the same bullet if you try to measure comparator A vs comparator B. Best to just pick one and stick with it.