Here are two different types of bullets, seated at the same CBTO but with different COAL. The bullets have been scratched by the comparator tool, which indicates the point on the bullet’s ogive where it will engage the rifling. Effects of Cartridge Overall Length (COAL), and Cartridge Base to Ogive (CBTO), Part 2.
Bryan Litz, Berger Bullets
Part One of this article focused on the importance COAL has in relation to SAAMI standards. This included magazine lengths and seating depths. Cartridge Base To Bullet Ogive Length is another measure of length that is important for precision. Figure 2. Figure 2.
Figure 2: Look at it. Imagine the bullet was seated so that the base of the nose (ogive), when the bolt is closed, just touches the beginning of riflings. This configuration is called touching the lands or touching the grooves, and it is an important measurement for precision hand loading. The distance that a bullet travels out of the case to engage the riflings can be critical to precision. This is due to the complex dynamics within internal ballistics, which occur in a blink of an eye. To optimize the accuracy of his handloads in a systematic way, it is critical that the precision hand-loader knows how to change the bullet seating depth relative to the barrel rifling. Understanding how to accurately and consistently measure the Cartridge Base To Ogive dimension (CBTO), is part of the knowledge required. This is explained in FULL ARTICLE. Bryan Litz provides a detailed discussion on how to accurately measure CBTO with different tools and methods. This includes the Hornady OAL gauge. This discussion can be found in the full article on the Berger Bullets site. Click Here to Read the Full Article. Why not use CBTO as SAAMI standard?
You may ask “Why isn’t CBTO listed in the SAAMI standard as well as COAL?”. There is a primary reason for this. It is due to the lack of standardization in the bullet nose shapes and measuring tools used to determine CBTO. Uniform CBTO: Benefits
When checking your COAL, there is another way to know your CBTO. This has to do with performance. You can achieve a CBTO of less than +/.001″ with good bullets, proper tooling, and well-prepared cases, but your COAL may vary up to.025″ (or more for other brands). This is not necessarily a bad thing, and it’s much better than the opposite. If your CBTO dimensions are varying but your COAL dimensions are tight (within +/.002″), then your bullet may be bottoming out in the seater cone at the bullet tip. This is a bad thing and should be avoided. It is normal that bullets can have the same nose shape but also have nose lengths ranging from.025″ to.075″. Summary of Cartridge Base to Ogive Discussion
Here are four key considerations for bullet seating depth in relation to CBTO. CBTO is an important measurement for handloaders to understand because it directly relates to precision potential. You can control it by setting bullet seating depth. 2. CBTO can be measured using a variety of tools and methods. Before you start, you should be aware of the pitfalls that can lead to inconsistent results. 3. A CBTO which produces the best accuracy in your rifle, may not produce the same precision in another rifle. Even if the rifle, bullets, comparator gauges and other components are identical. It’s possible the gauges may not be the same and measurements from one do not translate to another. 4. It is important to maintain a minimum variation in the CBTO dimension when creating quality handloads. This can be achieved by using high-quality bullets, proper tooling, and preparing the case mouths and necks properly for consistent seating. Click Here to Read the Full Article with More Information Article sourced from EdLongrange. We welcome tips and suggestions from our readers.
