CBTO vs seating depth

[Ned Ludd]

Jody - If the issue you're having is with regard to consistent seating depth (i.e. CBTO) with variance in bullet base-to-ogive length (and therefore variance in effective internal case volume), you might think of it in terms of velocity variance during a seating depth test. How far in/out can you seat a bullet over a given charge weight before the change in effective case volume gives you a detectable change in pressure/velocity? It's probably a lot farther than you might think relative to bearing surface length variance. I typically start charge weight testing with the bullet seated in the middle of the range I will most likely test later when optimizing seating depth.

For example, for a tangent ogive bullet like the 185 Juggernaut, I'd start them at .015" off the lands. After identifying and selecting an optimized charge weight, I will generally start seating depth testing from .003" off the lands to .027" off the lands in .003" increments. Note that .015" off the lands is exactly in the middle of this range, meaning the bullet will only be moving .012", or half the total range, in either direction after the charge weight has been set. In my hands, this is not a sufficient change in internal volume to detectably alter velocity or push the load out of the charge window, and yet I would consider a .012" variance in bearing surface length in a given lot of bullets unusually high.

I think you'll find that slight differences in bullet BTO will not have a measurable effect on pressure/velocity. However, if it's a real concern, you can always sort bullets BTO.

 

[hawkeyeshoooer]

Take a bullet in a sized but unpowdered, unprimed case; seat the bullet at least 0.100 longer than your expected COAL (base to tip).
Carefully chamber the cartridge, smoothly close the bolt, smoothly open the bolt and very carefully extract the cartridge. Do this at least 3 times until you get 3 consecutive measurements within 0.002. Measure either CBTO or CBTT or both. This tells you the COAL or CBTO that touches the lands. Subtract the desired jump distance and start loading ammo.

 

[Jody Simcox]

Take a bullet in a sized but unpowdered, unprimed case; seat the bullet at least 0.100 longer than your expected COAL (base to tip).
Carefully chamber the cartridge, smoothly close the bolt, smoothly open the bolt and very carefully extract the cartridge. Do this at least 3 times until you get 3 consecutive measurements within 0.002. Measure either CBTO or CBTT or both. This tells you the COAL or CBTO that touches the lands. Subtract the desired jump distance and start loading ammo.

Are you describing the method I saw in Alex wheeler’s vid? Without the ejector and firing pin? If not, am I doing anything between bolt closures or will this bullet move in a sized neck?

 

[Puzzaz01]

From Texas10.....Case length (neck length) differences will indeed affect neck tension. How well are you holding case length from base to shoulder (datum) and how much are you setting back the shoulder when you FL size? Or are you neck sizing only the twice fired brass?[/QUOTE]


Texas10 is right on the money with neck tension variation. Using a screw in die some times a tight neck will not fully seat a bullet with a consistent stroke of the press. If you want the consistent seating depth, try using an expander mandrel (like a a k&m) not the one in your sizing die along with an arbor press and a Wilson seater. You will see a noticeable difference.

Darrin

 

[Jody Simcox]

From Texas10.....Case length (neck length) differences will indeed affect neck tension. How well are you holding case length from base to shoulder (datum) and how much are you setting back the shoulder when you FL size? Or are you neck sizing only the twice fired brass?

Texas10 is right on the money with neck tension variation. Using a screw in die some times a tight neck will not fully seat a bullet with a consistent stroke of the press. If you want the consistent seating depth, try using an expander mandrel (like a a k&m) not the one in your sizing die along with an arbor press and a Wilson seater. You will see a noticeable difference.

Darrin[/QUOTE]
I’m FL sizing with honed neck from Forster (without the expander button) then expanding with a Sinclair mandrel .002” smaller than bullet diameter...

 

[Jody Simcox]

SFB and RRA issues.

Could someone help with this acronym? Can’t get a response from a PM....thanks

 

[Jody Simcox]

SFB and RRA issues.

rammac, have you had SFB and RRA issues? Are you still?