CBTO - Cartridge Base to Ogive

[Prerylyon]

CBTO is measure that can seem a bit confounding. Lots of articles and threads where its been praised or argued over. I wanted to share what I found to be one of the best explanations out there of what it is:

Effects of Cartridge Over All Length (COAL) and Cartridge Base To Ogive (CBTO) - Part 2

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.

bergerbullets.com

Using CBTO as a measurement to work up a load isn't required, but its worth being aware of what it is, and how it could be used. Everyone has a way to work up their cartridge length for best accuracy in their rigs; there is no one right way-many ways to get there; so certainly don't take this post as advice to change what already works well for you. At the end of the day, time at the range carefully testing your loads will get you there.

 

[Colorado Cowboy]

There are several things that you have to be aware of that were not mentioned in the article. One is freebore, the distance from the end of the chamber to the rifling. Some gun manufacturers like Weatherby use a lot of freebore. This can effect bullet accuracy no matter what CBTO you use.

Bottom line is that you have to find the right load that YOUR rifle likes.....and all are different.

 

[Prerylyon]

There are several things that you have to be aware of that were not mentioned in the article. One is freebore, the distance from the end of the chamber to the rifling. Some gun manufacturers like Weatherby use a lot of freebore. This can effect bullet accuracy no matter what CBTO you use.

Bottom line is that you have to find the right load that YOUR rifle likes.....and all are different.

No argument 'CC.

Despite my engineering background, CBTO and the bullet comparators/techniques to measure it were confusing. Bryan's article made it all click for me: its basically just a more precise datum to take a length measurement with a given bullet, given comparator gauges, and a specific rifle. Its not transferable from one person to the next because of all the variables; but the idea of a more precise datum from the CBTO vs COAL which is based on the bullet tip that may not be best datum; he explains well in his article.

I've been tinkering with CBTO and its been helpful for me after measuring "rifle seating" depth using this old school method:

https://www.nosler.com/blog/news-and-articles/2017/3/29/how-to-determine-proper-bullet-seating-depth-for-your-gun

I smoked my bullet with a match instead of the felt tip marker, on the advice of an older buddy, to darken it.

 

[Timr245]

There are several things that you have to be aware of that were not mentioned in the article. One is freebore, the distance from the end of the chamber to the rifling. Some gun manufacturers like Weatherby use a lot of freebore. This can effect bullet accuracy no matter what CBTO you use.

Bottom line is that you have to find the right load that YOUR rifle likes.....and all are different.

Exactly. With regards to the freebore, all Weatherby calibers have significant freebore, my 6.5-300 got way more accurate when I seated bullets closer to the lands.

Haha sorry CC I totally missed where you mentioned Weatherby’s freebore.

 

[Prerylyon]

Guys, my intent is just to share what I considered to be a good explanation of what CBTO is and how it can be used-if one so chooses. CBTO can be used to aid in determining best seating depth in a Weatherby rifle, or any rifle, for that matter; nor was I implying that there is a specific CBTO that will work great in a specific model of rifle or any rifle for that matter. Its just another way of measuring that jump to the lands-a way of measuring it that some folks like, and for a long time confused me...

 

[Colorado Cowboy]

I wasn't badmouthing your post, just adding info to it. Reloading and finding the load your particular gun likes is like an adventure. The deeper you get into it, the more you find out how little you knew when you started.

 

[Prerylyon]

I wasn't badmouthing your post, just adding info to it. Reloading and finding the load your particular gun likes is like an adventure. The deeper you get into it, the more you find out how little you knew when you started.

It's all good, 'CC.

You're right about that, its beyond satisfying when your groups start to finally tighten up and you realize just maybe your're starting to figure some of this stuff out.