CBTO measurement question

[notthattomsimonson]

I'm new to handloading and have a couple questions about the CBTO measurement. I bought the Hornady OAL gauge and bullet comparator set. Just had my Tikka rebarreled to 280 Ackley so I bought a few different bullets to try out along with a few boxes of factory ammo.

Using the Hornady tools, there was quite a bit of variation in the CBTO measurement between the bullets (I repeated the process 2 or 3 times for each bullet, these are averages):
Bullet 1: 2.68
Bullet 2: 2.70
Bullet 3: 2.77

Maybe I'm thinking about it wrong, but shouldn't the CBTO measurement be the same regardless of bullet? I thought that was the whole point of measuring to the ogive instead of looking at COAL. That 3rd bullet is a Barnes 150gr ttsx, which I know is a long bullet, but I didn't think that mattered for CBTO. Are these tools fairly inaccurate or maybe I'm not being consistent with how hard I jam the bullet into the lands? Anyone else have a similar experience?

2nd question relates to the factory loads: The CBTO measurement was 2.66, 2.70, 2.71, 2.70. If my chamber CBTO measurement (using the measurements above) is 2.70, does that mean that 3 of these 4 factory loads are going to jam into the lands when I try to chamber them? Just got my gun assembled and haven't been to the range yet to test it out, so maybe I'm just worrying about nothing?

 

[wind gypsy]

Yes, you are thinking about it wrong. People measure to the ogive because it’s a more consistent measurement than the tips of the bullet typically. It’s a better way to measure bullet distance from jam, that’s about the gist of it.

Factory ammo is generally loaded close to or under SAAMI max coal. The length of bullet noses varies widely, thus BTO measurement also varies widely.

 

[waspocrew]

Which projectiles are you measuring? 3 of the same brand/weight or 3 completely different projectiles (like Barnes, Hornady, and Berger)? The ogive will differ between brands and weights due to different projectile profiles.

 

[Pickettpuncher]

Which projectiles are you measuring? 3 of the same brand/weight or 3 completely different projectiles (like Barnes, Hornady, and Berger)? The ogive will differ between brands and weights due to different projectile profiles.

The measurement will be different with same brand and different style as well. Like the eldm vs eldx.

 

[EdP]

As I understand it, you are measuring CBTO to determine the value where the bullet is touching the lands in the chamber. I think that if the bullet comparator was an exact replica of your chamber that the way you are thinking about it would be correct. The measurement would be from the cartridge base to where the ogive diameter matched the chamber diameter at the lands and it would be the same regardless of bullet shape. There are a couple of problems with this. One is that the cartridge stops at the shoulder and the bullet is then pushed to the lands, locked in place, and CBTO is measured. Any base to shoulder difference between the case used to measure CBTO to the lands and a loaded case is an error in the measurement. Another problem is that the bullet comparator is not the same diameter as your lands, it is just something somewhat close. That makes the value you measure a "reference value" that you can use to accurately duplicate results, and not a true value.

I find that technique is important when using the Hornady tool to determine CBTO at the lands. As you mentioned, how hard you push the bullet in makes a difference so you need to be as consistent as possible. Also, how the case sits in the chamber may not be perfectly concentric. This can affect seating at the shoulder so I will take measurements at top dead center and 90 degrees in each direction, typically 3 at each and one additional for a total of 10. Any measurement that is wildly different I will toss and repeat. Looking at the 10, I will toss the high and the low and average the remaining 8. This is the process for every type bullet I am going to load.

The above is just what I do based on my understanding, or perhaps misunderstanding, for handloading hunting ammo. There are a lot of folks on here crafting ammo to a higher standard than me, with perhaps a better understanding than me, but I hope this helps.

 

[EdP]

Regarding your 2nd question, take a sharpie and black the bullet on one of each of your factory loads. Chamber the cartridge and remove it, then check to see if you have rifling marks. That will tell you if it was touching the lands when chambered.

 

[Jimbee]

Why don't seating dies seat from bullet ogive? Just something I think about sometimes.

 

[notthattomsimonson]

Regarding your 2nd question, take a sharpie and black the bullet on one of each of your factory loads. Chamber the cartridge and remove it, then check to see if you have rifling marks. That will tell you if it was touching the lands when chambered.

Good suggestion. My wife doesn't like it when I chamber live rounds at the house but I'll figure it out pretty quickly at the range.

 

[notthattomsimonson]

Which projectiles are you measuring? 3 of the same brand/weight or 3 completely different projectiles (like Barnes, Hornady, and Berger)? The ogive will differ between brands and weights due to different projectile profiles.

3 different. Hornady CX, Hornady BTSP and Barnes TTSX. The TTSX is a significantly longer bullet, so I figured the ogive profile was messing with where the Hornady comparator tool seated. I think I understand it now.

 

[BBob]

Why don't seating dies seat from bullet ogive? Just something I think about sometimes.

Well they do (The ogive is the curved front end of the bullet no?) they typically touch way forward of throat/land touch point. The problem with a seating stem that would contact the bullet near touch point is that it would be very thin and prone to split and crack. This is somewhat common with say a Redding VLD stem. They are prone to cracking.

 

[Jimbee]

Well they do (The ogive is the curved front end of the bullet no?) they typically touch way forward of throat/land touch point. The problem with a seating stem that would contact the bullet near touch point is that it would be very thin and prone to split and crack. This is somewhat common with say a Redding VLD stem. They are prone to cracking.

Maybe I'm not understanding what the ogive is? I thought it was the circumference of the bullet that contacts the lands?

 

[BBob]

Maybe I'm not understanding what the ogive is? I thought it was the circumference of the bullet that contacts the lands?

No, the ogive is defined as the complete portion of the bullet in front of the bearing surface.

 

[notthattomsimonson]

As I understand it, you are measuring CBTO to determine the value where the bullet is touching the lands in the chamber. I think that if the bullet comparator was an exact replica of your chamber that the way you are thinking about it would be correct. The measurement would be from the cartridge base to where the ogive diameter matched the chamber diameter at the lands and it would be the same regardless of bullet shape. There are a couple of problems with this. One is that the cartridge stops at the shoulder and the bullet is then pushed to the lands, locked in place, and CBTO is measured. Any base to shoulder difference between the case used to measure CBTO to the lands and a loaded case is an error in the measurement. Another problem is that the bullet comparator is not the same diameter as your lands, it is just something somewhat close. That makes the value you measure a "reference value" that you can use to accurately duplicate results, and not a true value.

I find that technique is important when using the Hornady tool to determine CBTO at the lands. As you mentioned, how hard you push the bullet in makes a difference so you need to be as consistent as possible. Also, how the case sits in the chamber may not be perfectly concentric. This can affect seating at the shoulder so I will take measurements at top dead center and 90 degrees in each direction, typically 3 at each and one additional for a total of 10. Any measurement that is wildly different I will toss and repeat. Looking at the 10, I will toss the high and the low and average the remaining 8. This is the process for every type bullet I am going to load.

The above is just what I do based on my understanding, or perhaps misunderstanding, for handloading hunting ammo. There are a lot of folks on here crafting ammo to a higher standard than me, with perhaps a better understanding than me, but I hope this helps.

Thanks for this explanation, it makes sense to me now. I was having trouble with a couple of the bullets sticking in the hornady modified case, which made it difficult to know if I was "kissing" the lands, or had to push harder to get the bullet to slide in the case, so that was part of my variability. The long ogive profile of the ttsx probably made it stick sooner in the comparator base, hence the longer reading. I wasn't thinking about how the difference between the diameter of the comparator and my actual lands would affect the measurement.

One related question though. Barnes says to load the 150 gr TSX to a COAL of 3.27. Based on the measurements I did, that would be a 90 thousandths jump, which is way longer than I would have started. But now I feel like I should ignore the CBTO measurements for the reasons mentioned above and just go with the Barnes COAL?

 

[BBob]

Maybe I'm thinking about it wrong, but shouldn't the CBTO measurement be the same regardless of bullet? I thought that was the whole point of measuring to the ogive instead of looking at COAL.

No. The comparator bushing will contact forward of the actual bullet land touch point. Every bullet model has a different curve to its ogive so there's where the differing bullet variances lie. If you used a bushing made from the chamber reamer used on your gun from a stub from your barrel then and only then would you have truly meaningful results. That is why it is called a comparator, it compares things and doesn't actually measure things or at least the things you are attempting to measure. Subtle difference.

 

[waspocrew]

Thanks for this explanation, it makes sense to me now. I was having trouble with a couple of the bullets sticking in the hornady modified case, which made it difficult to know if I was "kissing" the lands, or had to push harder to get the bullet to slide in the case, so that was part of my variability. The long ogive profile of the ttsx probably made it stick sooner in the comparator base, hence the longer reading. I wasn't thinking about how the difference between the diameter of the comparator and my actual lands would affect the measurement.

One related question though. Barnes says to load the 150 gr TSX to a COAL of 3.27. Based on the measurements I did, that would be a 90 thousandths jump, which is way longer than I would have started. But now I feel like I should ignore the CBTO measurements for the reasons mentioned above and just go with the Barnes COAL?

In my experience with Barnes, I find the distance to the lands, then back off .050”.

 

[EdP]

Handloaders typically work with CBTO to try to find a bullet jump that gives the best accuracy for a particular bullet and firearm. The COAL is important if a cartridge is going to be fed from a magazine because a cartridge with the bullet loaded too far out may not fit in the magazine. COAL doesn't matter for a single shot rifle, only CBTO. SAAMI specifies a COAL for each cartridge so mfgs of firearms and ammo have a standard that should ensure all factory ammo works in all factory firearms chambered for that cartridge. As such, it is independent of the bullet used. Firearm mfgs can produce firearms with magazines that will accommodate a longer COAL without violating the standard, so you may be able to load longer without an issue.