COAL - The Bastard Stepchild of Precision

[Jager]

So, you painstakingly set up your seating die to exactly where you want it. After nudging the micrometer on top a click or two, the calipers finally read exactly, precisely, what you had decided upon.

You smile, happy in the knowledge that this uber precise load has every chance to be special.

Without touching the seating die, you reach over and gently pick up another charged case from the loading block. Running it up in the press, the bullet makes its way down into the neck. You pick up the calipers again... and the smile melts away from your face.

It's three-thousandths longer than the one just before.

Wondering if maybe you mis-read that first one, you seat a third cartridge. This one is one-thousandth short. Sigh.

Welcome to the world of COAL - Cartridge Overall Length. We've all been there.

There's a better way, of course. Hang around handloaders much and you'll hear CBTO - Cartridge Base to Ogive a lot more often than you do COAL. At least where the discussion is about the where the bullet is with respect to our rifle's lands, a critical dimension for many. CBTO measurements can be a lot more precise than COAL.

I mostly use a third measure... CSTO - Cartridge Shoulder to Ogive. It shares the same inherent precision as CBTO, but without the fiddliness of those comparators that latch onto our calipers.

Yesterday I ran a seating depth exercise. Sixteen increments, two rounds each. Bullet was a Berger .30-caliber 115gr FB Target. Not as good as a custom bullet, but really quite good.

As I loaded each pair of rounds, I measured them carefully for both COAL and CSTO...





The COAL Variance and CSTO Variance in each case was the measured difference between the two rounds in that set. No surprise that COAL wandered all over the place like a drunken sailor, even with just a sample size of two. But perhaps a little bit of a surprise how superior the CSTO measurements were. CBTO, measured carefully, would have been similarly exact.

Alas, COAL isn't going anywhere. You can share COAL with a buddy. Or put it in a book and some fellow fifty years from now will know exactly what you meant. Not so with CBTO or CSTO.

But in a world where we spend inordinate amounts of time peering into the depths of arcana, of getting stuff as close to zero as we possibly can... every little thing matters.

Use CBTO or CSTO wherever you possibly can.

 

[Beiruty]

Bullet overall length vs Bullet to Ogive

 

[1bamashooter]

OAL is measured to see if they will fit in your magazine BTO for everything else. I try to tell all these new Reloaders on FB not to measure COAL every time I see one doing it

 

[Bill Norris]

Jager said - Use CBTO or CSTO wherever you possibly can. I agree CBTO is the measurement of most importance. O K I'll bite. How do you measure CSTO ?

 

[newbieshooter]

Jager said - Use CBTO or CSTO wherever you possibly can. I agree CBTO is the measurement of most importance. O K I'll bite. How do you measure CSTO ?

this was in the bulletin last year:

Accuracy One Seating Depth Comparator Review « Daily Bulletin

Looking for a faster, easier, and more accurate way to measure bullet positions on your loaded rounds? Check out the Accuracy One Seating Depth Comparator.

bulletin.accurateshooter.com

 

[Straightshooter1]

My goal is to seat my bullets to the same depth within all the cartridges, regardless of bullet's BTO or OAL. The amount of volume in the cartridges needs to be as consistent as possible as variances can be substantial as we can see with tuning our loads by adjusting seating depth by .003" or so to get the best possible groupings. Using CBTO as a measurement for this has a lot of room for error due to the variance between the contact point of the comparator insert used for that measurement and the contact point of the seating stem. I find the CBTO only useful for a starting point for a distance to the lands; that's all. COAL is useful for determining the maximum length available to loading cartridges in a mag. So, I sort bullets by the bullet's base to the seating stem contact point, which gives me the best consistency in seating depth. Sorting by BTO or AOL just doesn't cut it. And since I don't touch or jam the lands, CBTO doesn't really serve any effective function.

 

[David Christian]

My goal is to seat my bullets to the same depth within all the cartridges, regardless of bullet's BTO or OAL. The amount of volume in the cartridges needs to be as consistent as possible as variances can be substantial as we can see with tuning our loads by adjusting seating depth by .003" or so to get the best possible groupings. Using CBTO as a measurement for this has a lot of room for error due to the variance between the contact point of the comparator insert used for that measurement and the contact point of the seating stem. I find the CBTO only useful for a starting point for a distance to the lands; that's all. COAL is useful for determining the maximum length available to loading cartridges in a mag. So, I sort bullets by the bullet's base to the seating stem contact point, which gives me the best consistency in seating depth. Sorting by BTO or AOL just doesn't cut it. And since I don't touch or jam the lands, CBTO doesn't really serve any effective function.

@Straightshooter1, not sure I follow your logic here. In my experience volume of cartridges is not what I am tuning with seating depth. Very much apples to oranges. Volume differences are negligible. Barrel exit timing is massive.

 

[Scott-ak]

Please provide the details on how you made your measurements and the tools that you used.

Why tools? The same reason 1bama counsels fb users on coal. The measurement tool matters... a lot. The very best mitutoyo calipers are good to +/- .001. Folks that use these digital calipers and post .000x numbers don't understand or simply *want* to believe they are that accurate. Wanting just doesn't make it true. If they were I'd never pick up my mics!

Let me give an example. You are going for a .002 shoulder bump and you measure .002. The caliper manufacturer says at the very best the measurement is .001 to .003. It doesn't matter how good you think you are with the calipers or how many times you can repeat the measurement. The manufacturer has said, at best, your reading is +/- .001!

If you are using some of the built for purpose tools like the wilson case gage depth micrometer excellent! They are more accurate tho I am not convinced and have not seen independant literature, like ansi, confirming that hey are accurate to the 10 thousanth.

 

[Straightshooter1]

@Straightshooter1, not sure I follow your logic here. In my experience volume of cartridges is not what I am tuning with seating depth. Very much apples to oranges. Volume differences are negligible. Barrel exit timing is massive.

I agree about the importance of barrel exit timing.

We do know that changes in case volume for given powder charge changes the pressure curve along with the acceleration curve. This changes the barrel exit timing. It has a bigger effect than the change in overall distance when changing seating depths. This is what I see when I examine the calculations that QuickLoad makes. Because barrel exit timing is tied to the pressure curve, I don't understand how you see this as apple to oranges . . .???

 

[Jager]

... We do know that changes in case volume for given powder charge changes the pressure curve along with the acceleration curve...

Pressure change between increments is negligible. As you march slowly towards the lands, with increasing COAL/CSTO, you will see pressure and velocity fall off a tiny amount with each load. That then reverses when the ogive begins to make contact with the lands. You begin to see velocity/pressure rise at that point.

Note that QuickLoad has no notion of where the lands are, so the chart below assumes bullet jump for every load. But it's easily observable in the field. Your chrono will tell you when land contact is made as actual velocities begin to head north from those predicted.





Bill, yes, there are a couple of tools out there for measuring CSTO. Including the one linked by newbieshooter.

 

[Enosiophobia]

The more I reload I feel the less I try to sweat a thousandth here or there.

 

[wkdickinson]

I wish you would explain the tooling and how you measure CSTO, "without the fiddliness of those comparators that latch onto our calipers".

 

[rijndael]

I wish you would explain the tooling and how you measure CSTO, "without the fiddliness of those comparators that latch onto our calipers".

Seating Depth Comparator

The Accuracy One Seating Depth Comparator measures bullet seating depth, ensuring consistency. Compatible with 22 to 30 caliber cartridges.

bullettipping.com

 

[wkdickinson]

Seating Depth Comparator

The Accuracy One Seating Depth Comparator measures bullet seating depth, ensuring consistency. Compatible with 22 to 30 caliber cartridges.

bullettipping.com

Thanks!

 

[CharlieNC]

If we only understood why / how seating depth works when tuning????

 

[Bill Norris]

Pressure change between increments is negligible. As you march slowly towards the lands, with increasing COAL/CSTO, you will see pressure and velocity fall off a tiny amount with each load. That then reverses when the ogive begins to make contact with the lands. You begin to see velocity/pressure rise at that point.

Note that QuickLoad has no notion of where the lands are, so the chart below assumes bullet jump for every load. But it's easily observable in the field. Your chrono will tell you when land contact is made as actual velocities begin to head north from those predicted.

View attachment 1348917



Bill, yes, there are a couple of tools out there for measuring CSTO. Including the one linked by newbieshooter.

Yea, I saw that. I like the idea but I normally check each round right out of the die and place appropriately in my box in sequence order of difference so I'm not sure the device would really be worth the expense.

 

[Ned Ludd]

My goal is to seat my bullets to the same depth within all the cartridges, regardless of bullet's BTO or OAL. The amount of volume in the cartridges needs to be as consistent as possible as variances can be substantial as we can see with tuning our loads by adjusting seating depth by .003" or so to get the best possible groupings. Using CBTO as a measurement for this has a lot of room for error due to the variance between the contact point of the comparator insert used for that measurement and the contact point of the seating stem. I find the CBTO only useful for a starting point for a distance to the lands; that's all. COAL is useful for determining the maximum length available to loading cartridges in a mag. So, I sort bullets by the bullet's base to the seating stem contact point, which gives me the best consistency in seating depth. Sorting by BTO or AOL just doesn't cut it. And since I don't touch or jam the lands, CBTO doesn't really serve any effective function.

Altering the seating depth of jumped bullets by as much as +/- .015" affects effective case volume and pressure by so little as to be practically unmeasurable in terms of the effect on velocity by typical chronographs.

Further, sorting bullets by base-to-seating stem contact point will not necessarily have any effect at all in terms of improving consistency of seating depth. The two critical contact points for improving seating depth consistency are 1) where the seating die stem contacts the bullet nose just below the meplat, and 2) where the caliper insert use to measure CBTO seats on the bullet ogive just above the bearing surface (see cartoon below). Selecting the bullet base as one of the length sorting contact points means you have unnecessarily introduced additional length variance from both the boattail and bearing surface regions into the length sorting process. Sorting bullets base-to-ogive, or base-to-seating stem contact may improve the consistency of the amount of bearing surface contacting the case neck wall. Whether this provides significant benefit in a given application will need to be determined by the user.

Most of the issues referred to above can be minimize by sorting bullets between the two critical contact points (i.e. where the seating die stem contacts the bullet nose just below the ogive, and where the caliper insert use to measure CBTO seats on the bullet ogive just above the bearing surface). Tools such as Bob Green's Comparator do exactly this and are designed to improve seating depth consistency. It likely also improves the consistency of COAL measurements, but that may depend on the specific type and Lot# of bullets sorted with it (i.e. is there significant length variance present in the region between the seating die stem contact point and the bullet meplat?).

I sort bullets by OAL for the purpose of obtaining uniform points with a pointing die. In doing so, I have also found that for the bullets I use, there is little variance in the boattail or bearing surface lengths relative to that found within the nose region. Thus, sorting the bullets I use by OAL is also a bit like a "poor man's" Bob Green Comparator, in that if most of the OAL variance within a bullet is in the nose region, sorting by OAL will likely also improve length consistency between the two critical contact points.

If the issues described above with respect to COAL/CBTO variance or seating depth consistency are troubling anyone, the notion of length sorting bullets using a tool such as the Bob Green Comparator is worth consideration. Alternatively, one might start by sorting bullets using OAL just to see whether there might be any improvement, as we all have calipers so it typically won't cost anything but time to length sort a few bullets.

 

[MSS]

I’ve been using CBTO for years. I am definitely intrigued by CSTO. Great thread.

 

[rjtfroggy]

I’ve been using CBTO for years. I am definitely intrigued by CSTO. Great thread.

I've done the same, but the last time I loaded I used the Accuracy one tool and I will never go back. I believe it is more accurate, definitely faster and much easier to do.

 

[Dave M.]

Seating Depth Comparator

The Accuracy One Seating Depth Comparator measures bullet seating depth, ensuring consistency. Compatible with 22 to 30 caliber cartridges.

bullettipping.com

I use this tool religiously. It is super easy to use and very repeatable. I set it to zero using a master “dummy” round and then load all rounds to +/-.0005”