Effects Of Cartridge Over All Length (COAL) And Cartridge Base To Ogive (CBTO) – Part 2
So now you know the dimension from your bolt face to where your bullet touches the rifling using your bullets and gauge. Now all you have to do is adjust your seating die so that it makes ammo that matches this dimension. At this dimension you are "touching the rifling". It is from this starting point that you can tune your rifle very effectively (read HERE for Berger's recommended process to optimize seating depth for precision). Whatever amount you make this dimension longer than this starting measurement is a jam. If the starting dimension was 2.110" and you change it to 2.120" you are using a .010" jam. If you change your dimension to 2.090" you are using a .020" jump (as an example).
The key to this situation is that you find which CBTO shoots best in your rifle using your bullets and your gauge. It will take a bit of effort to get this sorted out especially if you haven't done it before but I can assure you that once you know and control this dimension accurately you will be able to obtain better performance from your load in your rifle. It is important to remember that you can't ignore COAL especially if you want to feed through a magazine. However, if you know your CBTO and then find out your COAL is 0.050″ too long to feed through a magazine then you are equipped to decide what to do next. If you must feed through the magazine then you will know for sure that you will have to add 0.050" more jump (make the CBTO 0.050" shorter) to get the cartridge into your magazine. This may or may not affect the rifles performance but at least you will have hard and reliable data from which to make decisions.
Benefits of having a uniform CBTO
There is another aspect to knowing your CBTO when checking your COAL as it pertains to performance. With good bullets, tooling, and carefully prepared cases you can easily achieve a CBTO that varies less than +/- .001" but your COAL can vary as much as .025" extreme spread (or more with other brands). This is not necessarily bad and it is much better than the other way around. If you have a CBTO dimension that varies but your COAL dimension is tight (within +/- .002) then it is most likely that your bullet is bottoming out inside the seater cone on the bullet tip. This is very bad and is to be avoided. It is normal for bullets to have precisely the same nose shape and it is also normal for these same bullets to have nose lengths that can vary as much as .025".
Figure 6. Zooming in on a bullet's meplat reveals irregularities which are normal and prevent consistent COAL measurements.
This variation in nose length typically does not negatively affect performance. The reason this is true is because as long as the nose shape is the same from bullet to bullet, the only way a nose length variation will negatively affect performance is if this variation in length has a significant impact on the outside diameter of the meplat (pronounced MEE-plah) or tip of the bullet. When it comes to Berger bullets we purposefully set our dies so that a variation in nose length has essentially no impact on meplat diameter. The way we do this is proprietary but it is effective and makes normal nose length variation essentially irrelevant.
Summary of Cartridge Base To Ogive (CBTO)
To recap the important considerations regarding bullet seating depth as it relates to CBTO, we can say:
• CBTO is a critical measurement to understand for handloaders because it's directly related to precision potential, and you control it by simply setting bullet seating depth.
• Tools and methods for measuring CBTO vary. Most have pitfalls that you should think carefully about.
• A CBTO that produces the best precision in your rifle may not produce the best precision in someone else's rifle. Even if you have the same rifle, same bullets, same model of comparator gauges, etc. It's possible that the gauges are not actually the same, and measurements from one don't translate to the same dimension for another.
• Once you find the CBTO that produces the best precision in your rifle, it's important to allow minimal variation in that dimension when producing quality handloads. This is achieved by using quality bullets, tooling, and properly preparing case mouths and necks for consistent seating.
Bryan Litz majored in Aerospace Engineering at Penn State University and worked on air-to-air missile design for 6 years in the US Air Force before taking a job as Berger Bullets Chief Ballistician in November 2008. Bryan has been an avid long range shooter since the age of 15. In particular, Bryan enjoys NRA Long Range Prone Fullbore/Palma competition and is the current National Palma Champion. Bryan is also a husband and proud father of 3.
The key to this situation is that you find which CBTO shoots best in your rifle using your bullets and your gauge. It will take a bit of effort to get this sorted out especially if you haven't done it before but I can assure you that once you know and control this dimension accurately you will be able to obtain better performance from your load in your rifle. It is important to remember that you can't ignore COAL especially if you want to feed through a magazine. However, if you know your CBTO and then find out your COAL is 0.050″ too long to feed through a magazine then you are equipped to decide what to do next. If you must feed through the magazine then you will know for sure that you will have to add 0.050" more jump (make the CBTO 0.050" shorter) to get the cartridge into your magazine. This may or may not affect the rifles performance but at least you will have hard and reliable data from which to make decisions.
Benefits of having a uniform CBTO
There is another aspect to knowing your CBTO when checking your COAL as it pertains to performance. With good bullets, tooling, and carefully prepared cases you can easily achieve a CBTO that varies less than +/- .001" but your COAL can vary as much as .025" extreme spread (or more with other brands). This is not necessarily bad and it is much better than the other way around. If you have a CBTO dimension that varies but your COAL dimension is tight (within +/- .002) then it is most likely that your bullet is bottoming out inside the seater cone on the bullet tip. This is very bad and is to be avoided. It is normal for bullets to have precisely the same nose shape and it is also normal for these same bullets to have nose lengths that can vary as much as .025".
Figure 6. Zooming in on a bullet's meplat reveals irregularities which are normal and prevent consistent COAL measurements.
This variation in nose length typically does not negatively affect performance. The reason this is true is because as long as the nose shape is the same from bullet to bullet, the only way a nose length variation will negatively affect performance is if this variation in length has a significant impact on the outside diameter of the meplat (pronounced MEE-plah) or tip of the bullet. When it comes to Berger bullets we purposefully set our dies so that a variation in nose length has essentially no impact on meplat diameter. The way we do this is proprietary but it is effective and makes normal nose length variation essentially irrelevant.
Summary of Cartridge Base To Ogive (CBTO)
To recap the important considerations regarding bullet seating depth as it relates to CBTO, we can say:
• CBTO is a critical measurement to understand for handloaders because it's directly related to precision potential, and you control it by simply setting bullet seating depth.
• Tools and methods for measuring CBTO vary. Most have pitfalls that you should think carefully about.
• A CBTO that produces the best precision in your rifle may not produce the best precision in someone else's rifle. Even if you have the same rifle, same bullets, same model of comparator gauges, etc. It's possible that the gauges are not actually the same, and measurements from one don't translate to the same dimension for another.
• Once you find the CBTO that produces the best precision in your rifle, it's important to allow minimal variation in that dimension when producing quality handloads. This is achieved by using quality bullets, tooling, and properly preparing case mouths and necks for consistent seating.
Bryan Litz majored in Aerospace Engineering at Penn State University and worked on air-to-air missile design for 6 years in the US Air Force before taking a job as Berger Bullets Chief Ballistician in November 2008. Bryan has been an avid long range shooter since the age of 15. In particular, Bryan enjoys NRA Long Range Prone Fullbore/Palma competition and is the current National Palma Champion. Bryan is also a husband and proud father of 3.
