Fired or Sized Case When Measuring COAL

[daved]

I use a Sinclair tool to measure max COAL and calculate max CBTO. The instructions say to use a fired case and that's what I've always done. It occurred to me recently that a fire formed/cleaned/annealed/sized case might provide a more accurate value since such a case would fit the chamber and position the ogive more precisely at the lands. Any thoughts?

Thanks for input,

DaveD

 

[Supersubes]

The error will be equal to the amount of headspace you have between the sized case and the chamber. Pretty small number, especially if you typically Jump bullets. I think a fired case might give you an error in the other direction, if it doesn’t sit all the way down into the chamber, which is difficult to verify. I use a prepped and ready to load case.

 

[daved]

My thought was like yours - fully prepped case will fit the chamber better.

 

[502Chevelle]

Which ever you choose, be constant in your method. Don’t use new brass to set it up and then use fire formed brass to shoot. I would set it up for all of your new brass, and once it’s fired one time, check it again. You may need to make a slight adjustment, or maybe you will get lucky, and it will be the same.

 

[deanq]

Only use a sized case. Sizing is what makes 'em grow.

 

[Snuby642]

Sizing for a bolt gun -002
For a gas gun -004
On the shoulders, (bump).

The neck before trimming grows in lenght .

None of that sets coal or cbto.

Your stressing over 002 to 004 jump difference. You may get 002 difference in each box of bullets in ogive and lenght.

 

[marchboom]

You're measuring the distance between the case base and the bullet ojive. It really doesn't matter what case you use as long as the case chambers easily and doesn't have to be forced into the chamber. I use a fired case, drilled and tapped for the Hornady chamber measuring tool. When using this tool I take 10 measurements and use the average. Just don't jam the bullet into the lands. That would alter the measurement. If you "jam" the bullet (during the seating portion) the "unjammed" measurement will give a guide as to how much.

 

[bghartm]

My recommendation is to used resized brass but it probably doesn't matter.

 

[Snuby642]

What @marchboom said.

A fired case from your gun to find the lands.
Then your sized brass and cbto will always be within a couple of thousands.

Probably not many can shoot good enough groups to see that difference,
But someone will probably blow through 100 rounds on the chrono to prove it exists.

At the end of the day it's not a chrono competition.