CBTO totally confused

[quacksmoker]

So I am clueless to this CBTO. I am loading some hunting rounds using Nosler Accubond for a hunting rifle. I sized down my fireformed brass .002 and lee collet sized the neck lightly just to hold a bullet and took several measurements jamming the bullet in to get an average rough estimate. I then took that rough estimate and set up my redding competition seater die to seat the bullet to that rough measurement. I took some blue dykem and marked the bullet and made fine adjustments with the mic seater to see if the rifling will still contact the bullets and also by pressing the cartridge lightly into the chamber to see if it will stick. With some fiddling i found the CBTO for this bullet. I then took a Nosler Partition and use the same method but i have found the Partition does not have the same CBTO as the Accubond. The difference in CBTO between the two is .056”, both 160 grain 7mm. And then I loaded the Partition to the Accubond’s CBTO and the bullet stuck in the rifling and bolt will not close. My confusion is that shouldnt the CBTO be a fixed measurement since it is a base to ogive and should be the same for all bullet accross the board? Or I am i tripping out?

 

[Lone Hunter]

Most all Bullets will have different CBTO measures as they all have different shapes. Be it large or small. You will have different measures say 175 SMK VS 168 SMK or 190 SMK. Same difference with the Bergers.

 

[retired]

in a word NO
BULLETS are shaped different, thus contact the lands at different lengths

 

[Dusty Stevens]

Those are 2 way different bullets. One is somewhat aerodynamic and one is shaped like a brick but is a killing machine

 

[mikecr]

CBTO is a measure from cartridge base to ogive.
When you set a specific CBTO, you're setting a datum to cartridge base -regardless of bullet used.
That is, any different bullets can be set to the same CBTO.
It has nothing directly to do with land contact.

Now if you want to achieve a particular land contact relationship, you can find the contact point and measure/log the CBTO of that round. You could reproduce any land relationship, with that bullet, using CBTO.
This is the utility in CBTO.

Changing bullets means changing bullet nose shapes, so both contact relationship and CBTO will change with that. You can't simply reproduce a land relationship between different bullets by setting the same CBTO.

 

[johnfred1965]

The ogive datum on your bullet is where the bullet diameter drops below the full diameter of the shank. You can set the CBTO the same with different bullets, but depending you how sharply or shallowly the nose points up, you will get contact with the lands at different lengths. This is what everyone else said, just worded a little different if that helps.

 

[Texas10]

If the tool you were using to measure CBTO was an exact replica of your chambers throat/lands then and only then would you be able to set all bullets to the same measurement. But the tool you are using bears no resemblance to your chamber. It is only a convenient tool to help you load consistently. There exists no standard describing the diameter or tolerance of the hole in the tool you are using, and because of this fact, you will get different measurements with different bullets, or even different lots of the same bullet.

The challenge is in consistently finding that magical "touch" dimension, keeping in mind that dimension is only a starting point to use in testing seating depth variations. Any method you use, as long as you can reliably repeat the process and get the same measurement is fine.

For me, I drill and tap 6-32 the flash hole of my fire formed case, and seat a bullet long and push it up the chamber using a portion of 3 piece cleaning rod. If it sticks upon retraction, it's in the lands. I then seat it a bit deeper and repeat until it no longer sticks upon retraction. I get reliable measurements to .001 with this method, and label and keep this "tool" in the box of bullets I'm using for die set up and future reference in case I am chasing the lands as the barrel wears.

This method will also tell you if you are properly sizing your brass by sliding only the brass into the chamber and noticing any tendency to stick.

 

[mikecr]

You were right before this:

The challenge is in consistently finding that magical "touch" dimension

There is nothing magical about it, it is fleeting with most cartridges, and changes from bullet to bullet with most brands of bullets (including same lot).

If it sticks upon retraction, it's in the lands. I then seat it a bit deeper and repeat until it no longer sticks upon retraction. I get reliable measurements to .001 with this method

Your bullets stick in lands, or not, within 1thou of each other.. Pure BS..

This method will also tell you if you are properly sizing your brass by sliding only the brass into the chamber and noticing any tendency to stick.

So you use a magical, incredibly precise bullet/land sticking point, to evaluate head spacing.. Even worse BS..

I don't know whats wrong with me. But I just can't let this stuff go without response.

 

[Texas10]

Tell me how you had problems when you tried the procedure I outlined and I'd be happy to help. Should be able to get you on the right track very quickly.

 

[270WinDude]

There's at least 1 post regarding this issue a page or 3 back on this forum. I'm not there observing your method but I can say I've got decent results doing similar tests before I had a comparator and they were all close with a couple exceptions. Also those are 2 pretty different bullet types, as mentioned different bullets will contact rifling at different points. First time I noticed I had a 150gr softpoint which was way shorter then my VLD handloads but the ogive measurement was significantly longer. I couldn't figure out why this stubby 150gr bullet wasn't getting stuck into my rifling. Your conducting a measurement where the BULLETS SHAPE is a significant variable, you're not measuring the inside of your chamber.