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!

 

[Straightshooter1]

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!

Don't worry about comparing COAL's as there can be significant differences even within the same box and even more from lot to lot of bullets. Mag length may be your only COAL limit to be concerned with. Just focus on the CBTO for your starting point and you'll do just fine.

 

[Old Navy]

Do you have the #26 insert in your tool?

 

[Rob01]

Do the OAL gauge a few times to make sure the bullet is getting to the lands. If you are at 2.5” for an oal from base to tip of bullet then there is something wrong with the rifle’s chamber.

 

[Old Navy]

The BTO should be around 2.295 with the the 142 SMK.

 

[DirtySteve]

I think you might be having an issue with something but it is really hard to figure out with out being there while you are using the tool.

First I would like to suggest that while sliding the modified case and bullet into the chamber you are very careful. When it is in the chamber gently push the grey rod forward to see if the bullet was pushed back in the case while inserting it. The use the locking screw to holdthe rod in place while extracting the case from the chamber. Also check on line for a video to help you figure out the issue if you have not figured it out.

 

[Straightshooter1]

The BTO should be around 2.295 with the the 142 SMK.

That would assume his 140 SMK's have the same BTO as the 142 SMK's to get the same CBTO.

And will add, there's a difference in OAL's between the two of ~.075".

 

[Ned Ludd]

If you wish to measure COAL, then measure COAL directly using your caliper jaws only (i.e. not using a comparator/insert). Trying to estimate COAL from a CBTO value taken with a comparator/insert may be misleading for a number of reasons. The aluminum Hornady caliper inserts I own have a much smaller hole than other brands such as Sinclair that are presumably intended for the same caliber bullets. As such, instead of seating down onto the bullet ogive at a point just barely above the top of the bullet bearing surface, the Hornady inserts may seat noticeably farther out on the bullet ogive toward the meplat as compared to some other brands. That alone may render the CBTO measurement much different than one taken with a different comparator/insert. Further, any COAL value estimated from such a CBTO measurement may be inconsistent with published book values.

One major caveat associated with comparing CBTO values between different reloaders is that there is no guarantee that someone else's caliper insert has the same diameter hole as yours. Likewise, one major caveat associated with comparing COAL values is that bullet OAL can vary by as much as .020" to .030" between different Lot #s, or even within the same Lot #, potentially making a valid comparison of COAL measurements at a given CBTO difficult.

 

[dellet]

The gauge has a certain learning curve to develop a feel for when the bullet touches the lands.

Start the old fashioned way, seat a bullet in an empty, sized case at 2.810 and see if it chambers. If it does, then it absolutely confirms operator error.

You can also build a test dummy round by seating a bullet out to 2.950” and keep reducing length until the bolt closes. Then you will have a known length for your chamber and can work out the feel of pushing a bullet out that far.

I can say without a doubt, you’re not the first person who has felt false contact of the bullet stopping, when it was basically just resistance.

Only other piece of advice would be to make sure the chamber is absolutely clean.

 

[Benchrest Braxton]

When using this tool I jam the bullet as hard as I can into the rifling, of course the bullet sticks in. This is what I call my "jam" measurement and work back from that.

 

[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. It wasn’t an estimation!

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!

 

[Coyotefurharvester]

I use the rod and 2 set/lock collars method. I purchased some 36" acrylic 1/8" rods and 1/8" set screw collars off Amazon(cheap). Now I can verify my Hornady tool numbers, if I see "strange" numbers. If I start inserting the tool with the bullet deep inside the modified case, I get inconsistent results. Start with the bullet ogive outside the modified case, but short of the expected coal. Then all you have to do is keep the tool straight after shoulder on case makes contact with the chamber shoulder(most important).

 

[Laurie]

The chamber spec for the 6.5 Creedmoor has a throat of 0.2645" diameter, ie only a half thou greater than the nominal bullet diameter. I have several makes/models of 6.5mm match bullets that equal or exceed that figure. This results in Hornady OAL gauge/modded case users thinking they have reached the lands when in fact it is the bullet encountering resistance passing through the throat. I very quickly gave up on this tool with the cartridge when I shot it in its early days for this very reason, and adopted the previously advised method in Post #9:

The gauge has a certain learning curve to develop a feel for when the bullet touches the lands.

Start the old fashioned way, seat a bullet in an empty, sized case at 2.810 and see if it chambers. If it does, then it absolutely confirms operator error.

You can also build a test dummy round by seating a bullet out to 2.950” and keep reducing length until the bolt closes. Then you will have a known length for your chamber and can work out the feel of pushing a bullet out that far.

I can say without a doubt, you’re not the first person who has felt false contact of the bullet stopping, when it was basically just resistance.

Only other piece of advice would be to make sure the chamber is absolutely clean.

ie making up an over-length inert round and chambering / attempting to chamber it. In fact I use this method for all my rifles with gunsmithed minimum-SAAMI spec chambers anyway, as I decided long ago that the Stoney Point / Hornady tool isn't very reliable in such chambers, the modified case often not fitting the chamber perfectly.

A good illuminated magnifying glass is invaluable in setting COALs with this method alongside making up multiple inert rounds (such a glass is very handy for other handloading tasks too). The glass lets you see very fine, barely visible rifling marks on the bullet ogive when you get the inert rounds' COALs down to lightly touching. It's difficult to feel this through the bolt on chambering, especially if you don't go the effort of completely stripping the bolt body. (The reason for the multiple inert rounds is to use one with a 'clean' bullet at each new, deeper setting to identify the marks solely for that setting.)

 

[K22]

Book COAL's do not always match a specific rifle / bullet combo. For example, the Sierra Book test COAL for the 85 BTHP in a 243 Win will not chamber in my Browning X Bolt but will chamber in my Rem 700.

This is why I always check the MAX base to ogive for any bullet I select to use in a given rifle. I cannot comment on the tool in question. I use the Frankfort Arenal tool (homemade) version, which is simple, fairly accurate and repeatable. It does not require a "special" case. I measure at least 3 bullets from a given lot and use the longest as my max length. I have used this method for years and it works quite nicely and is as easy as it can get.

One of the best methods I have used is to run a fired case in that rifle in a neck sizing size to size about 1/8" of the neck. I seat the bullet long then slowly chamber the case into the rifle chamber to push the bullet into the lands which results in a max jam length. I start load development .020" back from that. The reason I don't use this anymore, just for a spot check, is that it engraves the bullet. I don't like wasting bullets.

 

[Rob01]

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. It wasn’t an estimation!

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!

Yup if bullet not sliding freely it can cause issues. The tool is super easy to use. 2.5” is way short and that explains it. Fix that and you should be able to find oal pretty easy.

 

[gunsandgunsmithing]

I've never seen CBTO in any manual. The books show max OAL to the tip of the bullet. CBTO is Cartridge Base To Ogive. Two very different things and yes, different measurements. Maybe I'm missing something.

 

[Laurie]

Maybe I'm missing something.

I believe you are. The OP is saying he used the Hornady OAL measurement gear and apparently ascertained the CBTO value for the bullet reaching the lands. However, a bullet seated so resulted in a 2.5" proper COAL, obviously wrong.

So either the rifle has a massively wrong chamber, ie no freebore, or the CBTO reading he obtained is incorrect. @dellet and I believe the latter as this issue (short CBTO readings) is common in properly cut 6.5CM chambers as the very limited bullet-throat clearance gives a false impression of the ogive having contacted the lands with some bullets.

 

[Coyotefurharvester]

Book COAL's do not always match a specific rifle / bullet combo. For example, the Sierra Book test COAL for the 85 BTHP in a 243 Win will not chamber in my Browning X Bolt but will chamber in my Rem 700.

This is why I always check the MAX base to ogive for any bullet I select to use in a given rifle. I cannot comment on the tool in question. I use the Frankfort Arenal tool (homemade) version, which is simple, fairly accurate and repeatable. It does not require a "special" case. I measure at least 3 bullets from a given lot and use the longest as my max length. I have used this method for years and it works quite nicely and is as easy as it can get.

One of the best methods I have used is to run a fired case in that rifle in a neck sizing size to size about 1/8" of the neck. I seat the bullet long then slowly chamber the case into the rifle chamber to push the bullet into the lands which results in a max jam length. I start load development .020" back from that. The reason I don't use this anymore, just for a spot check, is that it engraves the bullet. I don't like wasting bullets.

This works well, if you strip the bolt. No need to strip the bolt when using a rod set collars. Easy to try multiple bullets , bullet types only need to more one collar.

 

[Bill Norris]

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. It wasn’t an estimation!

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!

The point where the bullet actually rest against the lands is hard to find if the bullet is dragging inside the modified case. The bullet needs to slide freely without any drag period.

Forget about "book" COAL. That is only a reference for the particular given load at the time of testing. Your loads will most likely not match up with it.

 

[Reformedcitykid]

Thanks to everyone for their input! To clarify, I’m not referencing COAL trying to match it, I know that every rifle is different. That being said, it rang alarm bells when I was .3 shy. The rifle has been an absolute tack driver to date, so I don’t think it’s an issue with the chamber or barrel. As per my previous comment, I think it’s an issue with the modified case.

When I get home I’ll try to run it through an expander and/or polish and sand the throat and report back.