COAL using CBTO from Hornady OAL gauge significantly shorter than book COAL?

[reformedcitykid]

Hi all,
I just got a Hornady OAL gauge and am trying to find the CBTO for my Tikka T3x CTR 6.5cm rifle using a Hornady 6.5cm Modified Case & SMK 140gr bullets. As per Sierra's book, the COAL is 2.810". When I measure the CBTO using the modified case I get 2.0080", which gets me a COAL of 2.5125. This seems significantly shorter than the book COAL.

Has anyone else encountered this? Any thoughts would be much appreciated.

Thanks!

 

[FEENIX]

Hi all,
I just got a Hornady OAL gauge and am trying to find the CBTO for my Tikka T3x CTR 6.5cm rifle using a Hornady 6.5cm Modified Case & SMK 140gr bullets. As per Sierra's book, the COAL is 2.810". When I measure the CBTO using the modified case I get 2.0080", which gets me a COAL of 2.5125. This seems significantly shorter than the book COAL.

Has anyone else encountered this? Any thoughts would be much appreciated.

Thanks!

No, I have never had any issues measuring CBTO/COAL. Show us a picture of how you are measuring your COAL and CBTO.

 

[waspocrew]

What exactly are you measuring? Are you measuring the distance to the lands in your chamber?

 

[FireFlyFishing]

Hi all,
I just got a Hornady OAL gauge and am trying to find the CBTO for my Tikka T3x CTR 6.5cm rifle using a Hornady 6.5cm Modified Case & SMK 140gr bullets. As per Sierra's book, the COAL is 2.810". When I measure the CBTO using the modified case I get 2.0080", which gets me a COAL of 2.5125. This seems significantly shorter than the book COAL.

Has anyone else encountered this? Any thoughts would be much appreciated.

Thanks!

So, it's as easy as @FEENIX's second video from Brownells. The key is having the bullet comparator tool coupled with OAL tool. I like to do five different bullets and take the avg of all the measurements. If I have a measurement way different than the other four I'll remeasure that bullet in the OAL tool. 99% of the time you're within .001-.002". I usually seat 0.020" deeper than my "jam" measurement I got from the OAL tool.

The load books typically will give you the SAAMI spec COAL. This is what factory ammo seating depth to chamber in all SAAMI spec'd chambers.

Edit: The exception is lathe turned bullets. You can do COAL for your final measurement.

 

[MagnumManiac]

I found the easiest and quickest way to find the rifling and then measure the CBTO is to cut 2 1mm slits in the neck, size the case, carefully put it in the chamber, close the bolt lifting and lowering the handle a few times with the ejector removed, then carefully remove it. Measure both OAL & CBTO after this. I normally do this a few times to average results, but rarely does this method alter much.
CBTO is different to COAL, maybe this is your confusion?

Cheers.

 

[FEENIX]

I found the easiest and quickest way to find the rifling and then measure the CBTO is to cut 2 1mm slits in the neck, size the case, carefully put it in the chamber, close the bolt lifting and lowering the handle a few times with the ejector removed, then carefully remove it. Measure both OAL & CBTO after this. I normally do this a few times to average results, but rarely does this method alter much.
CBTO is different to COAL, maybe this is your confusion?

Cheers.

My GS made this for me for my .338 Thor,

 

[Mikecr]

As per Sierra's book, the COAL is 2.810"

No, your COAL is whatever YOU choose, given local components (chamber throat, bullets).
Nobody at Sierra knows what your choices are.

 

[reformedcitykid]

Fellas, thanks for the input! To clarify, I measured CBTO using the Hornady OAL gauge, then removed the comparator from the calipers and measured COAL while still attached to the OAL gauge, so I'm not confusing the 2 numbers. Yes, I am trying to find the lands. And yes, I understand that book COAL is an estimation and every rifle is different, which is why I'm trying to find my rifles' measurements to go off of. However it is a good basic reference number and being .3 off raised some flags.

However, after fiddling with it a bit more and using a few different types of bullets, I think the issue lies with the modified case. It seems there is too much neck tension causing the bullet to meet what feels to be the lands. The bullets are not able to fall freely from the OAL if I turn it upside down. I'm going to sand/polish the inside of the neck to see if that works and will report back. Thanks!

 

[FireFlyFishing]

Fellas, thanks for the input! To clarify, I measured CBTO using the Hornady OAL gauge, then removed the comparator from the calipers and measured COAL while still attached to the OAL gauge, so I'm not confusing the 2 numbers. Yes, I am trying to find the lands. And yes, I understand that book COAL is an estimation and every rifle is different, which is why I'm trying to find my rifles' measurements to go off of. However it is a good basic reference number and being .3 off raised some flags.

However, after fiddling with it a bit more and using a few different types of bullets, I think the issue lies with the modified case. It seems there is too much neck tension causing the bullet to meet what feels to be the lands. The bullets are not able to fall freely from the OAL if I turn it upside down. I'm going to sand/polish the inside of the neck to see if that works and will report back. Thanks!

Interesting. All my modified Hornady cases are quite liberal…like no neck tension al all. Very loosely goosey.

 

[reformedcitykid]

Not mine at all. I can take a video of it when I get home but it sticks pretty good. I found the sticking point of the modified case, not the rifle haha

 

[KsKevin]

I've used both the 'Alex Wheeler' method (remove firing pin & ejector plunger) and the Erik Cortina method (hard jam with lighter neck tension) and while there is always a delta in the two methods, I have found both to provide consistent & repeatable results (when compared via doing multiple times 5x). The hard jam is usually 0.010 - 0.015" deeper as the bullet engraves into the rifling a bit. Now I've just moved to the hard jam method as it is simpler since you don't have to do the bolt disassembly. In the end, for me, it is just establishing a reference point as I begin seating depth testing during load development and for future assessments of throat erosion.

 

[FEENIX]

Fellas, thanks for the input! To clarify, I measured CBTO using the Hornady OAL gauge, then removed the comparator from the calipers and measured COAL while still attached to the OAL gauge, so I'm not confusing the 2 numbers. Yes, I am trying to find the lands. And yes, I understand that book COAL is an estimation and every rifle is different, which is why I'm trying to find my rifles' measurements to go off of. However it is a good basic reference number and being .3 off raised some flags.

However, after fiddling with it a bit more and using a few different types of bullets, I think the issue lies with the modified case. It seems there is too much neck tension causing the bullet to meet what feels to be the lands. The bullets are not able to fall freely from the OAL if I turn it upside down. I'm going to sand/polish the inside of the neck to see if that works and will report back. Thanks!

If your measured COAL in original post is 2.5125", that means your seating depth is way down there, compared to SAAMI 2.800". You need to measure your CBTO touching the lands and start .010-.020" of the lands. Something is definitely wrong. Re-visit how you are measuring, and go from there.

 

[reformedcitykid]

If your measured COAL in original post is 2.5125", that means your seating depth is way down there, compared to SAAMI 2.800". You need to measure your CBTO touching the lands and start .010-.020" of the lands. Something is definitely wrong. Re-visit how you are measuring, and go from there.

Yes I know, that was the whole point of the original post... I was trying to find the lands, realized something was wrong, and was asking for input.

As per my follow-up post, I think the issue was with the modified case. I am going to sand/polish the inside of the neck and see if that helps. Thanks

 

[asd9055]

Yes I know, that was the whole point of the original post... I was trying to find the lands, realized something was wrong, and was asking for input.

As per my follow-up post, I think the issue was with the modified case. I am going to sand/polish the inside of the neck and see if that helps. Thanks

Just use your expander on your "depriming die"