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!

 

[TheOfficeT-Rex]

Something's up with the way you're using the tool. I always found the hornady tool a bit fiddly to get right.

There's no way you should be coming up .3 short.

 

[BuildingConceptsllc]

Did you make sure of the insert that you're using is the same insert they are using?

 

[JoshPutman]

So with the bullet touching the lands you are coming up with an overall length .3" shorter than the book calls for?

 

[Threadcutter308]

For giggles, write down what YOU THINK the Inside Diameter of the insert SHOULD be, then measure it. I think you'll be very surprised at what you find.

Hint; I've seen ID's of inserts be as much as 0.005" smaller in diameter than the diameter of the bullet(s) they are designed/manufactured to measure. 0.001" difference in insert ID makes a huge difference in Cartridge Base To Ogive measurements.

I recommend forgetting all about trying to get that particular measurement to jive up with Sierra's number (or anyone else's for that matter).

Just find your lands with a dummy round, measure the Cartridge Base To Ogive of that dummy round, make a note of it and now you have your point of reference going forward.

(By way of example) If you want to jump your bullet/loaded rounds 0.020", adjust your seating die so that when you're measuring your newly loaded rounds, the new measured value of Cartridge Base To Ogive is your original noted length, minus 0.020". Simple. That's all you have to do.

 

[PinesAndProjectiles]

Are you comparing CBTO to OAL?

Because that would probably be around .300” different.

 

[TheOfficeT-Rex]

I think the focus should be on the use of the OAL gauge, and not the fact a comparator is a comparator and is probably a little off, or because he used CBTO to get to COAL.

He shouldn’t measure the lands and come up .3 shorter than book COAL.

OP needs to remeasure his lands, and possibly by using a different method.

 

[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.

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!

 

[johnnyvw]

Stupid question: are you re-zeroing the caliper between the CBTO and OAL?

 

[Reformedcitykid]

Stupid question: are you re-zeroing the caliper between the CBTO and OAL?

Yes! I’m not the sharpest tool in the shed, but I did remember to do that haha

 

[Baron23]

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.

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!

It is my experience that you need to get a good feel for when it "touches the lands" and I did find that at first I was coming up short from some small bit of resistance prior to hitting the rifling. I also take about 5-10 measurements and have found that outliers from my screwing up the technique were very apparent and that the rest of the set matched up very well.

YMMV

 

[Rio Precision Gunworks]

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!

Im not familiar with Sierra's book. So, couple questions.

Does the book give you a CBTO measurement to go along with the COAL?

Does the book state that with the COAL of 2.810 and an SMK 140gr that it is touching the lands?


Also, try using the stripped bolt "wheeler method" and see how that does compared to what you're doing at the moment.

 

[Thump_rrr]

Im not familiar with Sierra's book. So, couple questions.

Does the book give you a CBTO measurement to go along with the COAL?

Does the book state that with the COAL of 2.810 and an SMK 140gr that it is touching the lands?


Also, try using the stripped bolt "wheeler method" and see how that does compared to what you're doing at the moment.

No CBTO but pretty comprehensive.
You can find Sierra load data online for most popular cartridges they sell bullets for.
https://sierrabullets.files.wordpress.com/2017/01/6-5-creedmoor-load-data-112817.pdf

 

[Reformedcitykid]

Gents, glad to report back with some good news! As suspected, it was an issue with the modified case neck. I sanded it down until the bullet slid freely, and took 10 measurements:
2.3230"
2.3270"
2.3260"
2.3275"
2.3270"
2.3260"
2.3250"
2.3230"
2.3245"
2.3260"
For an average of 2.3256" aka 2.3255"

Any recommendations how far off the lands to start loading? I've read from .01" to .04", if anyone has a compelling argument one way or another please let me know!

Once again, thanks for everyone's input!

 

[straightshooter1]

Gents, glad to report back with some good news! As suspected, it was an issue with the modified case neck. I sanded it down until the bullet slid freely, and took 10 measurements:
2.3230"
2.3270"
2.3260"
2.3275"
2.3270"
2.3260"
2.3250"
2.3230"
2.3245"
2.3260"
For an average of 2.3256" aka 2.3255"

Any recommendations how far off the lands to start loading? I've read from .01" to .04", if anyone has a compelling argument one way or another please let me know!

Once again, thanks for everyone's input!

I always like to start at .010" off, or if using a mag, then maximum that'll fit the mag.