Reloading Question - COAL vs Measurements

[treillw]

If I have a cartridge with a COAL of 3.625" and I want to determine what the cartridge will measure with a comparator when it is that long I take the COAL - bergers published "nose length" for the bullet?

So for a .30 caliber 215 gn hybrid target I take 3.625" - .872" = 2.753"

In other words a cartridge which measures 2.753" with my comparator will have a COAL of 3.625"? Correct?

Thanks!

 

[FEENIX]

If I have a cartridge with a COAL of 3.625" and I want to determine what the cartridge will measure with a comparator when it is that long I take the COAL - bergers published "nose length" for the bullet?

So for a .30 caliber 215 gn hybrid target I take 3.625" - .872" = 2.753"

In other words a cartridge which measures 2.753" with my comparator will have a COAL of 3.625"? Correct?

Thanks!

Berger has a nice article on COAL vs CBTO that might help you better understand the concept.

 

[treillw]

Berger has a nice article on COAL vs CBTO that might help you better understand the concept.

I read it all. So in other words there is no way for me to figure out the dimension until I get the dummy round my smith provided me with and I should just load some preliminary rounds measuring 3.625" to the tip of the bullet (3.625" COAL)?

Why don't they make all comparator bodies for .308" bullets with the same hole diameter? They would then all bear on the the bullet at the same nose diameter and effectively be standardized (if they have their comparator holes perfectly reamed).

 

[FEENIX]

I read it all. So in other words there is no way for me to figure out the dimension until I get the dummy round my smith provided me with and I should just load some preliminary rounds measuring 3.625" to the tip of the bullet (3.625" COAL)?

Why don't they make all comparator bodies for .308" bullets with the same hole diameter? They would then all bear on the the bullet at the same nose diameter and effectively be standardized (if they have their comparator holes perfectly reamed).

No idea how you're measuring or what your set-up is but below is how I simply measure mine; .300 WM with 200g NAB ...

COAL measurement.

Zero the comparator.

CBTO measurement.

 

[treillw]

My problem is that I need to calculate the CBTO measurement only having the COAL measurement. Is that doable?

 

[FURMAN]

Great pictures FEENIX. In my experience the numbers on Berger's website are not close enough to be loading ammo with. There are tolerances and I would prefer to have exact measurements for the ammo I shoot. For example the bearing surface, as measured with my Stoney Point, does not match the numbers on Berger's site.

 

[MudRunner2005]

My problem is that I need to calculate the CBTO measurement only having the COAL measurement. Is that doable?

Yes, but you will need a comparator tool, and a set of calipers.

And to find out your distance to the lands you will need an OAL gauge and modified cartridge gauges.

Hornady makes both of these tools.

Comparator: Hornady Lock-N-Load Bullet Comparator Basic Set 6 Inserts

OAL tool: Hornady Lock-N-Load Overall Length Ga Bolt Action

Modified cartridge gauges: hornady overall length gauge modified case - MidwayUSA

 

[FEENIX]

I read it all. So in other words there is no way for me to figure out the dimension until I get the dummy round my smith provided me with and I should just load some preliminary rounds measuring 3.625" to the tip of the bullet (3.625" COAL)?

Why don't they make all comparator bodies for .308" bullets with the same hole diameter? They would then all bear on the the bullet at the same nose diameter and effectively be standardized (if they have their comparator holes perfectly reamed).

If you read ...

Effects of COAL and CBTO | Part 1 | Berger Bullets Blog
Effects of COAL and CBTO | Part 2 | Berger Bullets Blog

esp. part 2, it tells you need to have some kind of tools to measure as noted by Mud which exactly what I have, all of them.

 

[LaHunter]

To answer the OP's question: In my opinion trying to calculate CBTO based on COAL is not wise reloading. I guess you could attempt it, but it is "Best Practice" to actually MEASURE CBTO. For example, you could load 10 rounds with the same CBTO length and most of these rounds could possibly have different COAL measurements, using the same bullets. I personally use the Hornady Comparator and it works well for me. I am sure there are other brands that work just fine as well. You are sorta 'flying blind' without a comparator of some type to measure CBTO. I recommend you buy one. They are very inexpensive for the value that you get from it.

Hope this helps and good luck

 

[treillw]

Thanks guys. I just loaded them to the COAL that the gunsmith gave me. I'll measure the CBTO off the dummy round he gives me once it arrives. Just want to have a few ready to go first thing

 

[just country]

morning, I am putting a stick in the mud. correct me if I am

wrong. each bullet maker starts the ogive(ojive) on a the bullet

depending on the weight of the bullet, whether match grade,

hunting or other at a precise place when mass producing the bullets.

a shooter can have a smithy cut the cartridge chamber to a specific

length. a shooter can have a short or long chamber to shoot

a specific makers bullet. the shooter can have the barrel maker to

produce a barrel to a specific twist to handle a specific weight

bullet. I had a 7mm weatherby made Hart barrel, straight contour,

#7 weight finished to 28in. SS the smithy ask me to produce

a dummy round to cut the chamber to a specific bullet length.

I shoot 145gr. TTSX bullets for the 7mm or .284cal.

instead of cutting the chamber to a standard length, the smithy

cut the chamber to a weatherby free bore length. I read the loaded

length of the cartridge. set my loading mic gauges to the dummy

cartridge length. brand new barrel shot .750 group.

now working on the precise powder charge, powder, and mic

length. THK U lightbulbgun)

Barnes shooter, no excuse for weatherby have 3. just me

 

[FEENIX]

Thanks guys. I just loaded them to the COAL that the gunsmith gave me. I'll measure the CBTO off the dummy round he gives me once it arrives. Just want to have a few ready to go first thing

Keep things simple and life easier, get those items listed (or similar) by Mud; no need to complicate things unnecessarily, esp. when you're rolling your own.

It becomes more evident when you're finding the sweet spot (seating depth/distance off the lands) for your load during development.

 

[Greyfox]

No idea how you're measuring or what your set-up is but below is how I simply measure mine; .300 WM with 200g NAB.

I also agree with this approach and tools used(although I also like the hex nut comparator from Sinclair).
One note: after you have established your load, I like to save a representative bullet and the exact ogive measurement for my chamber.. This will serve as a definitive method to monitor throat erosion of your chamber as the round count has increased down the road.