“CBTO vs. COAL Measurement Benefits”

 

This Week’s Tips or Tricks…

 

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Note: all below CBTO measurements noted were taken using a Hornady Bullet Comparator attached to a Hornady Dial Caliper as shown below.  Please also note that for all CBTO values documented below, I would also need to subtract 1.0035” from every CBTO measurement to get the corrected CBTO length.

 

Example: I measured CBTO with the Hornady Bullet Comparator attached to the Hornady Dial Caliper and noted the value as 3.79125”.  To find the length that you will need to subtract for your particular tool to get corrected CBTO, close the Dial Caliper jaws all the way down with the Bullet Comparator Tool and the correct size comparator inserted and document the value of the Dial Caliper when fully closed, in the case of my particular tool this value was 1.0035” when fully closed.  So to find corrected CBTO I would then calculate 3.79125” – 1.0035” = 2.78775” CBTO.  If you are using a digital caliper, simply close the jaws and press the ‘Zero’ button on your calipers to achieve your corrected CBTO.

 

 

                    

 

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Q:  Ok, what is the “Ogive” (pronounced oh-jyv) and why is it so important???

 

A:  Essentially the “Ogive” of the bullet (as a Reloader would refer to it and not Mr. Professor) is the section of the bullets outer body that will come into contact with the rifling lands FIRST…and now that lightbulb just got a bit brighter didn’t it and got those mice turning on that wheel of thought! 

 

First, I want you to walk over to your reloading bench, pull out your micrometer or caliper, get out a box of your favorite bullets you reload for hunting, target, or competition and I want you to just randomly select any (5) bullets from the box.  Now take each bullet and measure it from tip to end and write that value down on a piece of paper, then measure the next one and write it down, and so forth for all five bullets.  Now, I want you to look back at your piece of paper where you have those bullet lengths written down and tell me whether or not more than any (2) of those bullets even match EXACTLY!?  Typically with most ammunition (non-Match Grade), and especially bulk produced ammunition and bullets, the BEST tolerance you can expect is 0.003-0.005” variance and with cheaper mass production bullets as much as 0.005-0.015” variance in every bullets length!  The reason for this is that during the bullets manufacturing, molding, or even packaging processes, the bullets may have developed a slight flat spot on the tip, the copper or polymer tip may have even been worn down, deformed, or developed a lip or taper and eureka!…you now know why you have such a hard time trying to get your COAL of your reloads to stay consistent…because they simply aren’t ever going to be when using the manual’s method of measuring base to tip of the cartridge! 

 

Now, we all know COAL or OAL the acronym for “Cartridge Overall Length” which is measured from the very base of the casing to the very tip of the bullet (or technically referred to as the meplat).  So, what does CBTO stand for and how do we measure it???  CBTO is the acronym for “Cartridge Base to Ogive” which means that we are instead going to measure from the base of the casing to the Ogive of the bullet instead of the tip.

 

Now to see why CBTO is so much more accurate and as an example to prove this, here are the bullets that I measured…

 

.30 Cal Berger 168 grain VLD Hunting = 1.250”, 1.2485”, 1.2490”, 1.2485”, 1.254”

-Average Length = 1.250”

-Standard Deviation = 0.002074”

-Extreme Spread (ES) = 0.0055”

 

6mm Hornady 87 grain V-Max = 1.049", 1.0495", 1.04875", 1.049", 1.0525"

-Average Length = 1.0498”

-Standard Deviation = 0.001396”

-Extreme Spread (ES) = 0.00375”

 

.22 Cal Berger 80 grain VLD Target = 1.1155”, 1.114”, 1.11”, 1.112”, 1.114”

-Average Length = 1.1131”

-Standard Deviation = 0.001908”

-Extreme Spread (ES) = 0.0055”

 

Now at this point, you are just thinking…ok, so what that’s only an SD of 0.002-3” variance and an ES of 0.003-55”…that’s close enough for the girls I go with!  That may be so, but now let me show you what happens once you seat those bullets into a casing and we take some more measurements…

 

.30 Cal Berger 168 grain VLD Hunting [Tip-to-Base COAL] = 3.47”, 3.46825”, 3.477”, 3.46975”, 3.46725”

-Average Length = 3.4705”

-Standard Deviation = 0.003426”

-Extreme Spread (ES) = 0.00975”

 

.30 Cal Berger 168 grain VLD Hunting [Cartridge Base to Ogive (CBTO)] = 3.79125”, 3.793”, 3.79425”, 3.794”, 3.79325”

-Average Length = 3.7932”

-Standard Deviation = 0.001056”

-Extreme Spread (ES) = 0.003”

 

Now look at as those numbers grasshopper…when we measured using the Tip-to-Base method of COAL, we had an SD of 0.003” and an ES of 0.009”, but when we measured using the CBTO, now we are at an SD of 0.001” and an ES of only 0.003”….we just took 6 THOUSANDTHS of ES error out of play in our reloads by making one simple measurement change…because remember that errors COMPOUND onto each other and add up quick!

 

Now, for just an average run-of-the-mill RCBS FL Die Set…having an Ogive OAL SD of 0.0011”…that is good enough for now for the girl I go with!  Now, imagine throwing a top grade Die such as a Wilson into the mix here and we could be looking at a potential SD of less than 0.0005”!!!  When matches are won or lost by TEN-THOUSANDTHS…every THOUSANDTH counts!!!

 

When you start using a ‘bullet comparator’ tool for measuring the “CBTO” of the bullet…you will start to see that the MOST that a good bullet’s “Ogive” CBTO SD will vary is 0.001-2”!!!  Now for just a ~$50 investment you have just DOUBLED or even TRIPLED your cartridge COAL SD measuring consistency and imagine what this is going to potentially do for those MOA groupings on your next trip to the range!!!  

 

Second, why is it more accurate and consistent to measure from the “Ogive” instead of the tip of the bullet besides just the physical differences in the bullets?

 

If you reload using the Tip-to-Base of the cartridge method, COAL, we already know the lengths will vary as illustrated above and you will likely have a much higher SD and ES, but what else suffers majorly now because of this method now that we have seen the numbers…the initial “jump” of the bullet into the lands!  When you measure CBTO from the “Ogive” you are now measuring with a much lower SD and ES to ensure that all of the rounds are going to maintain a more consistent and repeatable “jump” distance into the lands which will provide a more uniform and accurate behavior when comparing bullet-to-bullet performance and shot groupings.  This will be especially crucial when dealing with secant ogive bullets that may be a real bear when it comes to touchy seating depths tolerances.  Also, by ensuring that the “jump” of each bullet is more consistent, you may also improve the consistency of your muzzle pressures and muzzle velocities because the initial gas expansion and pressures will be more uniformly and consistently distributed during each round (perfect world scenario here of course), having an even more desired potential effect of reducing your Velocity SD which means less MOA Elevation variance adjustments required down range!

 

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Q: You say I should measure from the “Ogive”, but why do manufacturers always suggest to measure COAL from the base of the casing to the tip of the bullet when referencing my Reloading Manuals?

 

A: The reason why they suggest measuring COAL instead of CBTO is primary due to liability and standardizing to SAAMI specs, but more so due to the fact that when you reload to CBTO the bullet may or may not fit/feed in your rifle or handgun magazine because it may exceed SAAMI specs!  The COAL is established in the reloading manuals to ensure adequate clearance and matching tolerances to the industry standard length of actions so that the “manual” can guide you to reloads that will fit in most any magazine or feeding device for your particular firearm.  When you reload using the CBTO method you are conducting what we refer to as “load development” which is specifically tailored to your particular firearm(s) for the best possible performance and accuracy with the components you have chosen to utilize.

 

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Do you have a Tip or Trick regarding reloading that you would like to see posted?  Please send us your Tip(s) or Trick(s) and we will see about getting them posted!

 

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Written By:  Kyle R. CEO of Broken Box R LLC

 

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