Effects Of Cartridge Over All Length (COAL) And Cartridge Base To Ogive (CBTO) – Part 2

[ADMIN]

The first half of this article focused on the importance of COAL in terms of SAAMI standards, magazine lengths, etc. There is another measure of length for loaded ammunition which is highly important to precision.

Refer back to Figure 2. Suppose the bullet was seated out of the case to the point where the base of the bullet's nose (ogive) just contacted the beginning of the riflings (the lands) when the bolt was closed. This bullet seating configuration is referred to as touching the lands, or touching the riflings and is a very important measurement to understand for precision handloading. Due to the complex dynamics of internal ballistics which happen in the blink of an eye, the distance a bullet moves out of the case before it engages the riflings is highly critical to precision potential. Therefore, in order to systematically optimize the precision of his handloads, it's critically important that the precision handloader understands how to alter bullet seating depth in relation to the riflings.
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This is a thread for discussion of the article, Effects Of Cartridge Over All Length (COAL) And Cartridge Base To Ogive (CBTO) – Part 2, By Bryan Litz. Here you can ask questions or make comments about the article.

 

[RegionRat]

Looks like the photo in Figure 5 was supposed to be the Stoney Point gage but was a copy of the bullet comparison.

Very good points on the differences between the reference tooling brass supplied with the Stoney Point versus the actual closed bolt distance. I have been in the habit of running the reference brass of the gage through my other case gages to get a feel of how far from nominal their supplied case measures. Then I compare it to my cycled cases. With a study of that difference, I very carefully construct a dummy round with those estimated offsets and attempt a study of your first method until I think I have verified the contact with the lands is real and not in my imagination. Access to a good stereo-microscope helps with the examination of the bullet, but tool room layout ink or magic marker works too.

Shallow reamer angles can make the contact points more or less difficult to determine. Based on the differences between reamer angles, do you have a favorite angle dimension? Say for .30 cal or .224, would you assume the same?

 

[TexSavage]

On the Stoney Point (Hornady) method, if instead of using the Stoney Point's case, you use a case that has been fired in your gun, and tap/thread the primer pocket to attach to the Stoney Point gauge will it then give as accurate a measurement as the first preferred method?

 

[MrBamboo]

I have several Hornady cases for measuring the distance from the base to ogive and use a comparator nut with caliper to get the distance. I thought this "worked" for some time until on one rifle the reloaded cartridges when inserted into the chamber made it hard to close the bolt. I then withdrew the unfired cartridge and fount that it was being jammed into the lands even though the Hornady measurement was "Good."

After some thought and going back to using COLA that I have been using since 1976. I realized that to correct this, the Hornady cartridge should be put into you action, and close the bolt to form it to your length, kind of like resizing the shoulder. Because their round is longer from the shoulder to the base which gives an inaccurate reading.

I am glad this reloading article did show some of these issues and gave another way to measure your cartridge.

Paul

 

[Whitesheep]

Measure many times

One thing not mentioned in Brian's excellent explanation is the "measure several times, cut once" axiom of building anything that requires exact measurements. I generally measure four different bullets of the same type from the same box three times each after verifying these four bullets vary less than 0.001" base to ogive in length. Then I take the average of these 12 measurements as the CBTO. It is interesting how some bullet brands/styles are more consistent than others when selecting the four bullets to use.

If you like this be sure to read the full Burger article as it has several other recommendations based on establishing the best CBTO and then moving on to powder weight.

I sincerely appreciate Brian and Berger sharing this information with the long range hunting community. It has really helped me develop better loans with Berger and other brands of bullets.

Thank you

 

[Mikecr]

It's easier to understand CBTO where we focus on just that.
Relationship to the lands doesn't matter with CBTO, nor do base-to-ogive measurements.
The best shooting CBTO will reveal itself through testing, regardless of anything else.
And it doesn't matter where the bearing to ogive junction is, as ogive contact datum is set by bore diameter and leade angle(before bearing).

Maybe I missed it in the article but another factor(as though there aren't enough) is ogive radius variance of bullet noses(even from the same type/lot). A change in this radius shifts all datums.
We can separate bullets into matching radius through measure(real or relative), which then qualifies the rest of our contact/datums.
I recommend this before acting off nose contact, like with meplat trimming, or determining local CBTO, etc.

 

[Alaska2006]

Guess I'm old school. Had a gent teach me this trick over 30 years ago. Take a fire formed case, knock the primer out, long seat a bullet with no primer, no powder and smoke the bullet with a match (bic lighter). Slide the beast in the pipe and close the bolt, you will see where the lands contact the smoke on the bullet when ejected, if the land grab and pull the bullet you will seen the line in the smoke where it pulled the bullet from the case. Works every time. I own the hornady gauges but seem to be a bit of a pain.

 

[barefooter56]

I have several Hornady cases for measuring the distance from the base to ogive and use a comparator nut with caliper to get the distance. I thought this "worked" for some time until on one rifle the reloaded cartridges when inserted into the chamber made it hard to close the bolt. I then withdrew the unfired cartridge and fount that it was being jammed into the lands even though the Hornady measurement was "Good."

After some thought and going back to using COLA that I have been using since 1976. I realized that to correct this, the Hornady cartridge should be put into you action, and close the bolt to form it to your length, kind of like resizing the shoulder. Because their round is longer from the shoulder to the base which gives an inaccurate reading.

I am glad this reloading article did show some of these issues and gave another way to measure your cartridge.

Paul

Mr Bamboo,
Paul,
This is why I always take the "dummy" round to the lands and check it by lightly polishing the tarnish off the bullet with 0000 steel wool and slowly re-chamber then extract it and check for lands marks on the bullet. If they are long I will seat the bullet a little deeper using very small adjustments on the seater stem , polish the marks off again and re-chamber and extract as before. I do this until I end up with 4 small "dots' or lines on the bullet. I am still slightly into the lands at this point but I still have a reference mark. Its at this point that I measure the CBTO with a bullet comparator and write that measurement along with the manufacturer , type and weight of bullet. I keep this round as a gauge to check throat erosion over the life of the barrel.
Hope this helps!

 

[Whitesheep]

The old school probably is just as good as I have used it for cartridge's Hornady doesn't have a modified case for. If I hadn't already invested in the tool I probably wouldn't now.

That being said for those of you who work up loads 1st with COAL to ogive and then for powder weight do you find a given rifle likes the same set back from the lands for different bullets or not? Is this different rifle to rifle? Just curious.

 

[barefooter56]

The old school probably is just as good as I have used it for cartridge's Hornady doesn't have a modified case for. If I hadn't already invested in the tool I probably wouldn't now.

That being said for those of you who work up loads 1st with COAL to ogive and then for powder weight do you find a given rifle likes the same set back from the lands for different bullets or not? Is this different rifle to rifle? Just curious.

whitesheep,
I have found that in some cases they do or its really close. If I had a CBTO off the lands of .010 for a Sierra .30 cal 155 grain bullet and wanted to try a Berger .30 cal 155 grain bullet. I would make a dummy round for the Berger 155 to the lands to find whats its CBTO has to be for it to be .010 off the lands and test there before starting on the lands and working back. Also If you use the Sinclair overall length tool. You use one of your once fired cases with it so you don't have to have a modified case like the Hornady/Stoneypoint tool requires.