CBTO Help Needed. 6.5 CM "Update" I Found The Lands
- Thread starter Mr Wick
- Start date
what is the overall length (base to tip) on the round? does it match or exceed book load data?
That looks really short to me, especially with a 140.
Work backwards.
Determine the distance to the lands. Set your bullet seating to the desired depth ( jump distance). Then you use the comparator to "measure" that round. Make note to always use same bullet and same insert for batches. Then you can verify your other rounds against that reading. The comparator reading will be potentially different for everyone, hence it should not be a "measurement" value like the OAL.
Do you have magazine restrictions as to length??
If so measure it and start there.
Now take a unprimed sized case and a bullet to make a dummy round to this length.
Will it chamber?
If not seat deeper and try chambering again. Repeat until it does smoothly
Cover bullet shank with Sharpie "black or blue" are my favorites for this operation. This will show your land marks easily and aid in determining how much to adjust seating die, bigger adjustments in the beginning, smaller adjustments as you start getting closer to your goal.
You'll need to wipe Sharpie off shank every couple tries to give you a clean reading.
Hope this helps.
Are you working with 1 set of calipers or 2?
If only 1 set im thinking you didn't zero after removing anvil.
dellet
Gold $$ Contributor
There are ways to verify your measurements. I’ll use some defaults since I don’t have yours.
JBM lists bullet length at 1.374”
Hodgdon lists case length at 1.910”
Your original COAL 2.625”
Original CBTO 1.995”
Case length + bullet length - COAL = seated depth of bullet.
1.910 + 1.374 = 3.284 - 2.625 = .659
Case length + bullet base to ogive - cartridge base to ogive = seated depth.
Since I don’t know bullet base to ogive
CTBO - case length + seated depth = bullet base to ogive
1.995 - 1.910 = .085 + .659 = .744
Bullet length - base to ogive = bullet tip
1.374 - .744 = .630
CTBO + tip length = COAL
1.995 + .630 = 2.625
So your bullet should be very close to .744” base to ogive with your insert, using your original numbers.
Your second length of 2.810” should have a CTBO 2.180”, based on a tip length of .630”
Is it possible you simply had a dyslexic moment reading the caliper?
Reading 2.812” instead of 2.182”?
JBM lists bullet length at 1.374”
Hodgdon lists case length at 1.910”
Your original COAL 2.625”
Original CBTO 1.995”
Case length + bullet length - COAL = seated depth of bullet.
1.910 + 1.374 = 3.284 - 2.625 = .659
Case length + bullet base to ogive - cartridge base to ogive = seated depth.
Since I don’t know bullet base to ogive
CTBO - case length + seated depth = bullet base to ogive
1.995 - 1.910 = .085 + .659 = .744
Bullet length - base to ogive = bullet tip
1.374 - .744 = .630
CTBO + tip length = COAL
1.995 + .630 = 2.625
So your bullet should be very close to .744” base to ogive with your insert, using your original numbers.
Your second length of 2.810” should have a CTBO 2.180”, based on a tip length of .630”
Is it possible you simply had a dyslexic moment reading the caliper?
Reading 2.812” instead of 2.182”?
6.5 forever
Silver $$ Contributor
This matches my number!
A two-part discussion by Berger Bullets on CBTO and COAL:
bergerbullets.com
bergerbullets.com
An article by Brian Litz, regarding CBTO and COAL:
That 1.995" measurement sounds fairly short to me, as well. But then, the key here is: every chamber and throat is different, from one rifle to the next.
On my own 6.5 Creedmoor rifles, my measured CBTO comes in around 2.2235". Nearly a quarter of an inch longer than your measurement on your rifle, with your tools. (Less the small amount of jump I generally use, subtracted fro that 2.2235".) I'm using the Hornady O.A.L. Gauge (the long rod tool) for the depth test along with a Hornady-supplied "modified" case, and the Hornady Bullet Comparator on Mitutoyo digital calipers.
Just to confirm -- You're using these two Hornady tools for CBTO measurement, yes?
Are you using the standard "modified" case that comes from Hornady, with the OAL gauge? Or is it one of your own Lapua cases that you had sent to Hornady for modification? (Or something else?)
When you screw-on the modified case to the OAL gauge, how firmly do you screw it on? (Meaning, if it's only weakly screwed-on, it might well be changing as you attempt subsequent measurements in the chamber.)
How snugly are you pushing the case+bullet on the OAL gauge into the chamber? Hornady's suggested amount of force is in the neighborhood of "3-4 lbs". If to the point of making minor scratching or even dents on the bullet, IMO that's too much force (which would yield a shorter CBTO length).
After pushing the OAL gauge to the lands, are you tightening the set-screw to ensure that length is captured for your calipers measurement?
When you remove the bullet away from the lands, are you using a dowel from the muzzle end to gently push back the bullet so you can remove the OAL gauge? (Or, are you using tapping or a bit of force, which might well be shoving the bullet deeper into the case before you actually get to the point of caliper measurement.)
When you measure the case-to-datum using the calipers+anvil, try several adjustments of that case on the calipers. Occasionally, the case can be slightly less than perpendicular off the jaws/anvil of the calipers, getting slightly different measurement results.
You indicated "30 times" ... Just to be sure, does this mean you went through the length testing inside the chamber followed by a handful of measurements of that length (with the calipers), and conducted this sequence 30 different times? Hornady also reminds that it is important to use the same technique from one test to another (as close as we can) ... the same amount of force for insertion, the same amount of dialing of the calipers, verification the case is perpendicular on the jaws/anvil, etc. Strange how seemingly minor variations, here, can end up with something other than a valid, consistent measurement.
And, once you get the CBTO values a ~dozen times and have decided on what value you're going use, you're then backing-off a certain "jump" amount from that value, yes, to use as your seating length? Is the 1.995" the straight CBTO or your CBTO-minus-jump value?
Hm. Beyond those verifications, I can't imagine what's going on.
Effects of Cartridge Over All Length (COAL) and Cartridge Base To Ogive (CBTO) - Part 1
Many shooters are not aware of the dramatic effects that bullet seating depth can have on the pressure and velocity generated by a rifle cartridge. COAL is also a variable that can be used to fine tune accuracy. It’s also an important consideration for rifles that need to feed rounds through a...
bergerbullets.com
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
An article by Brian Litz, regarding CBTO and COAL:
That 1.995" measurement sounds fairly short to me, as well. But then, the key here is: every chamber and throat is different, from one rifle to the next.
On my own 6.5 Creedmoor rifles, my measured CBTO comes in around 2.2235". Nearly a quarter of an inch longer than your measurement on your rifle, with your tools. (Less the small amount of jump I generally use, subtracted fro that 2.2235".) I'm using the Hornady O.A.L. Gauge (the long rod tool) for the depth test along with a Hornady-supplied "modified" case, and the Hornady Bullet Comparator on Mitutoyo digital calipers.
Just to confirm -- You're using these two Hornady tools for CBTO measurement, yes?
Are you using the standard "modified" case that comes from Hornady, with the OAL gauge? Or is it one of your own Lapua cases that you had sent to Hornady for modification? (Or something else?)
When you screw-on the modified case to the OAL gauge, how firmly do you screw it on? (Meaning, if it's only weakly screwed-on, it might well be changing as you attempt subsequent measurements in the chamber.)
How snugly are you pushing the case+bullet on the OAL gauge into the chamber? Hornady's suggested amount of force is in the neighborhood of "3-4 lbs". If to the point of making minor scratching or even dents on the bullet, IMO that's too much force (which would yield a shorter CBTO length).
After pushing the OAL gauge to the lands, are you tightening the set-screw to ensure that length is captured for your calipers measurement?
When you remove the bullet away from the lands, are you using a dowel from the muzzle end to gently push back the bullet so you can remove the OAL gauge? (Or, are you using tapping or a bit of force, which might well be shoving the bullet deeper into the case before you actually get to the point of caliper measurement.)
When you measure the case-to-datum using the calipers+anvil, try several adjustments of that case on the calipers. Occasionally, the case can be slightly less than perpendicular off the jaws/anvil of the calipers, getting slightly different measurement results.
You indicated "30 times" ... Just to be sure, does this mean you went through the length testing inside the chamber followed by a handful of measurements of that length (with the calipers), and conducted this sequence 30 different times? Hornady also reminds that it is important to use the same technique from one test to another (as close as we can) ... the same amount of force for insertion, the same amount of dialing of the calipers, verification the case is perpendicular on the jaws/anvil, etc. Strange how seemingly minor variations, here, can end up with something other than a valid, consistent measurement.
And, once you get the CBTO values a ~dozen times and have decided on what value you're going use, you're then backing-off a certain "jump" amount from that value, yes, to use as your seating length? Is the 1.995" the straight CBTO or your CBTO-minus-jump value?
Hm. Beyond those verifications, I can't imagine what's going on.
I read your post. Crazy.
1. Yes I am using all Hornaday products. Hornaday modified case.A two-part discussion by Berger Bullets on CBTO and COAL:
Effects of Cartridge Over All Length (COAL) and Cartridge Base To Ogive (CBTO) - Part 1
Many shooters are not aware of the dramatic effects that bullet seating depth can have on the pressure and velocity generated by a rifle cartridge. COAL is also a variable that can be used to fine tune accuracy. It’s also an important consideration for rifles that need to feed rounds through a...
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.
An article by Brian Litz, regarding CBTO and COAL:
That 1.995" measurement sounds fairly short to me, as well. But then, the key here is: every chamber and throat is different, from one rifle to the next.
On my own 6.5 Creedmoor rifles, my measured CBTO comes in around 2.2235". Nearly a quarter of an inch longer than your measurement on your rifle, with your tools. (Less the small amount of jump I generally use, subtracted fro that 2.2235".) I'm using the Hornady O.A.L. Gauge (the long rod tool) for the depth test along with a Hornady-supplied "modified" case, and the Hornady Bullet Comparator on Mitutoyo digital calipers.
Just to confirm -- You're using these two Hornady tools for CBTO measurement, yes?
Are you using the standard "modified" case that comes from Hornady, with the OAL gauge? Or is it one of your own Lapua cases that you had sent to Hornady for modification? (Or something else?)
When you screw-on the modified case to the OAL gauge, how firmly do you screw it on? (Meaning, if it's only weakly screwed-on, it might well be changing as you attempt subsequent measurements in the chamber.)
How snugly are you pushing the case+bullet on the OAL gauge into the chamber? Hornady's suggested amount of force is in the neighborhood of "3-4 lbs". If to the point of making minor scratching or even dents on the bullet, IMO that's too much force (which would yield a shorter CBTO length).
After pushing the OAL gauge to the lands, are you tightening the set-screw to ensure that length is captured for your calipers measurement?
When you remove the bullet away from the lands, are you using a dowel from the muzzle end to gently push back the bullet so you can remove the OAL gauge? (Or, are you using tapping or a bit of force, which might well be shoving the bullet deeper into the case before you actually get to the point of caliper measurement.)
When you measure the case-to-datum using the calipers+anvil, try several adjustments of that case on the calipers. Occasionally, the case can be slightly less than perpendicular off the jaws/anvil of the calipers, getting slightly different measurement results.
You indicated "30 times" ... Just to be sure, does this mean you went through the length testing inside the chamber followed by a handful of measurements of that length (with the calipers), and conducted this sequence 30 different times? Hornady also reminds that it is important to use the same technique from one test to another (as close as we can) ... the same amount of force for insertion, the same amount of dialing of the calipers, verification the case is perpendicular on the jaws/anvil, etc. Strange how seemingly minor variations, here, can end up with something other than a valid, consistent measurement.
And, once you get the CBTO values a ~dozen times and have decided on what value you're going use, you're then backing-off a certain "jump" amount from that value, yes, to use as your seating length? Is the 1.995" the straight CBTO or your CBTO-minus-jump value?
Hm. Beyond those verifications, I can't imagine what's going on.
2. It is screwed on very firmly.
3. Yes. I am tightening the screw.
4. The bullet comes out on its own. In the videos I've watched they say to gently slide the Overall Guage until you hit resistance. Bullet never gets stuck.
5. I have tried several different measuring methods.
6. Yes. I conducted the entire process 30 times. Beginning to end.
7. I have not subtracted the "jump" it's straight CBTO.
I'm alone today. I'll see if I can somehow rig a video up.
Thank you to everyone trying to help.
Have you ran any factory ammo through your new rifle?
I was curious if it felt like it had resistance, indicating that you might have a short chamber.
I was curious if it felt like it had resistance, indicating that you might have a short chamber.
Coyotefurharvester
Silver $$ Contributor
SAAMI cartridge length 2.700-2.825( you had a coal short of minimum)
Max case length 1.920 (so ogive should exceed that)
Chamber length 1.925-1.9408(SAAMI) so cbto will certainly exceed 1.925, probably 1.9408.
Without exceeding coal of 2.825(factory cartridge)
Bullet selection and the individual comparator body will determine what number you see. Make sure gage body are positioned to center the cartridge/case between caliper jaws(no canting).
6.5 cm generally have a generous freebore, this will definitely increase cbto and coal once you determine your rifles bolt face to jam length.
Max case length 1.920 (so ogive should exceed that)
Chamber length 1.925-1.9408(SAAMI) so cbto will certainly exceed 1.925, probably 1.9408.
Without exceeding coal of 2.825(factory cartridge)
Bullet selection and the individual comparator body will determine what number you see. Make sure gage body are positioned to center the cartridge/case between caliper jaws(no canting).
6.5 cm generally have a generous freebore, this will definitely increase cbto and coal once you determine your rifles bolt face to jam length.
Coyotefurharvester
Silver $$ Contributor
Size one case,no primer no powder. Seat a bullet to overall length 2.825. Measure cbto, record. Chamber in CLEAN rifle. Extract, check coal and cbto. It probably DIDN'T change, if it did(shorter because you pushed the bullet in deeper) that dummy cartridge bullet made contact with the bore(record new cbto). Seat bullet to 2.700 cartridge overall length. Measure cbto, record. Your best group will likely be between 2.700 cartridge length and max length.
DLT
Silver $$ Contributor
Huh do what ? There isn’t any converting. You have 1 round and on that round you have 2 measurements, a cartridge base to bullet ogive CBTO and a cartridge overall length COAL.
Upgrades & Donations
This Forum's expenses are primarily paid by member contributions. You can upgrade your Forum membership in seconds. Gold and Silver members get unlimited FREE classifieds for one year. Gold members can upload custom avatars.
Click Upgrade Membership Button ABOVE to get Gold or Silver Status.
You can also donate any amount, large or small, with the button below. Include your Forum Name in the PayPal Notes field.
To DONATE by CHECK, or make a recurring donation, CLICK HERE to learn how.
Click Upgrade Membership Button ABOVE to get Gold or Silver Status.
You can also donate any amount, large or small, with the button below. Include your Forum Name in the PayPal Notes field.
To DONATE by CHECK, or make a recurring donation, CLICK HERE to learn how.