CBTO Help Needed. 6.5 CM "Update" I Found The Lands

[dellet]

Try it backwards.

Seat the bullet out long, say 2.9, just enough bullet in the neck to not fall out. Slide it into the chamber and then collapse the comparator tool on the bullet. Make sure the bullet slides freely in the case and also the rod into the tool.

This will end up being a jam into the lands, but will give you a base line for length. Probably within .010” of touch.

The other thing to make sure of is that the modified case is the right length. CHeck base to shoulder datum line of the case against your brass and specs.

Last thing to make sure of is that the chamber is absolutely squeaky clean.

 

[Benchrest Braxton]

i put firm resistance on the oal tool and bullet, the bullet will stick inside the rifling so make sure you have a cleaning rod to tap it out.

i repeat a few times and as long as they are consistent i call that my jam point.

your results are incredibly short, any chance of a carbon ring in your throat? even then, your results are indicative of an almost freebore chamber

 

[SSchnd]

Wick for what its worth, My Savage Precision in 6.5CM is running a jump -20 thousandths and shooting an SD of 2-7 this past Sunday at muzzle. I do not have my book handy at work so I cannot quote the MV per test charges. I I believe I am running 2.210 Base to Ogive on my hornady tool. Book COAL is about 2.80-2.810 If I recall and the COAL is about that using Lapua SRP and H140ELDM. But I do not not use COAL as I measure base to ogive.

Your OP photo looks like a lot of bullet in the case. Where as my loads seems to be base of boattail right at the base of the neck to shoulder. case fill is about 88-90%.

Running about 1-2 thou neck tension. Bushing Sizer. 3rd loading of this brass and annealed after 2nd firing

 

[Dgd6mm]

Also as you push the bullet out of the lands try not to deform the tip. Be on the gentle side with the rod.

 

[Benchrest Braxton]

Also as you push the bullet out of the lands try not to deform the tip. Be on the gentle side with the rod.

not a huge deal imo as you are not taking any measurements from that point anyways…….but i also dont try and shoot the bullets i use to measure, they get get aside for future use with the tool

 

[pdog06]

I agree with Sigma. Firm push till it stops to get a consistent measurement. And like he said this will be a jam so take that into consideration when doing load development.

 

[Dgd6mm]

not a huge deal imo as you are not taking any measurements from that point anyways…….but i also dont try and shoot the bullets i use to measure, they get get aside for future use with the tool

No, but it is a plastic tip bullet.

 

[pdog06]

Matters if you’re measuring for them to fit in a magazine.
I usually make notes of both OAL and CBTO readings.

 

[Mdhmsmith]

I think you should get some confidence making SAAMI spec ammo before you move on to more advanced techniques. It won’t take long but you need to gain some experience loading to a specification. Gain familiarity with your tools in the process. We all started simple.

 

[Mr Wick]

After talking again with my new friend from the forum, we decided that the modified cartridge was way too tight and to take a wire cleaning brush to the inside of the neck. I did. After doing so, the bullet moved free and sweet. These are the new CBTO readings.

CBTO - 2.217, 2.217, 2.219, 2.220, 2.217, 2.218, 2.220, 2.217, 2.218, 2.219, 2.220, 2.217, 2.217 2.217, 2.217

COAL - 2.850

I determined the CBTO before the bump is 2.217. Would you all agree?

With my newly acquired and most likely accurate readings, do you think I should still measure to SAMMI. I've seen others post similar CBTO with Savage Elite Precision Rifles.

 

[piie]

CBTO = Cartridge Base To Ogive. There is zero in there to do with your rifle or your rifle’s chamber. The biggest variable here is where the ogive is defined on the particular bullet you have stuck in the case. Determining the actual ogive is just as difficult as determining the datum line on your case shoulder. That is why we don’t even try, and instead we just decide to be consistent in the way we measure and compare. If you want to compare measurements with others, using OAL will give you much better chance of repeatability since you simply measure tip to bottom with calipers. Yes, deformed tips can still be an issue, but if you do not ensure that additional information, like the make and insert numbers for the comparator used is supplied along with your cbto measurement and the bullet used, oal has a higher chance of being accurate.

 

[Dgd6mm]

^^^^^^^^^^^ This is good information.

 

[Bc'z]

CBTO = Cartridge Base To Ogive. There is zero in there to do with your rifle or your rifle’s chamber. The biggest variable here is where the ogive is defined on the particular bullet you have stuck in the case. Determining the actual ogive is just as difficult as determining the datum line on your case shoulder. That is why we don’t even try, and instead we just decide to be consistent in the way we measure and compare. If you want to compare measurements with others, using OAL will give you much better chance of repeatability since you simply measure tip to bottom with calipers. Yes, deformed tips can still be an issue, but if you do not ensure that additional information, like the make and insert numbers for the comparator used is supplied along with your cbto measurement and the bullet used, oal has a higher chance of being accurate.

I believe sorting bullets by over all length aid in having both COAL and CBTO measurements very consistent within a particular batch of ammo in relation to the lands.

 

[Kracken McGraw]

COAL - 2.850

If you came up with this from your measurement, I noticed Hornady recommended in their book to load the 140 Eldm bullet at 2.800 COL. So their recommendation would be a .050 jump to the lands, which sounds about right for a general place to start.

 

[Dgd6mm]

I believe sorting bullets by over all length aid in having both COAL and CBTO measurements very consistent within a particular batch of ammo in relation to the lands.

The one word in this sentence that jumps out to me the most is (consistent). Every aspect of reloading hinges on this for the utmost accuracy.

 

[JFrank]

After talking again with my new friend from the forum, we decided that the modified cartridge was way too tight and to take a wire cleaning brush to the inside of the neck. I did. After doing so, the bullet moved free and sweet. These are the new CBTO readings.

CBTO - 2.217, 2.217, 2.219, 2.220, 2.217, 2.218, 2.220, 2.217, 2.218, 2.219, 2.220, 2.217, 2.217 2.217, 2.217
Now Transferring CBTO 2.217 to COAL = 2.850

COAL - 2.850

I determined the CBTO is 2.217. Would you all agree? YES. If you wish to jump the bullet ( create a distance from the lands to the ogive) you would reduce the CBTO by way of adjusting/lowering the seating die stem. For example 2.200 would equal .017 jump

With my newly acquired and most likely accurate readings, do you think I should still measure to SAMMI. if you wish to load by magazine then yes sammi coal would be appropriate.

I've seen others post similar CBTO with Savage Elite Precision Rifles.

As a side note, Hornday has put a lot time into creating a cartridge that shoots very well at magazine length and published load recipes to go with that , efforts to improve on that may prove futile to a new reloader. I personally would suggest loading to sammi coal and get familiar with case sizing , case prep and charge testing followed by some bushing tests to determine the ideal bullet hold for your loads before moving further down the rabbit hole.