COAL - The Bastard Stepchild of Precision

[lgwatson]

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...


View attachment 1348840


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.

I do CBTO. How do you do CSTO?

 

[MSS]

Question on this. How does shoulder bump figure into CSTO?. Do I need to bump the shoulder every time on my bolt gun brass? I would guess so. I am set in my ways and am just now trying out minimum shoulder bump FL sizing every time. Generally, I neck size until I feel bolt closure resistance then I bump. My measurements are CBTO.

 

[Jager]

Question on this. How does shoulder bump figure into CSTO?. Do I need to bump the shoulder every time on my bolt gun brass? I would guess so. I am set in my ways and am just now trying out minimum shoulder bump FL sizing every time. Generally, I neck size until I feel bolt closure resistance then I bump. My measurements are CBTO.

CSTO simply measures from the shoulder of your case to the ogive of your bullet. If you bump your shoulder .002... that will be reflected in the measurement. If you then decide you want to increase your shoulder bump to .004... CSTO will then show that increased distance. And if your handloading process is largely oblivious to shoulder position - say, because you're following the die manufacturer instructions to screw down the die until it touches the shell holder; or because you mostly neck size - CSTO still works fine for measuring something like seating depth.

COAL, CBTO, and CSTO all essentially measure the position of the bullet with respect to the cartridge case. The advantage that CSTO and CBTO share is that they both are making their respective measurements across a generally well-controlled portion of that case/bullet combination. The area from the bullet ogive to the bullet meplat is often the area of greatest variability, the place where that case/bullet combination is sometimes not so well controlled, and that's why we typically see COAL numbers wander all over the place.

The singular benefit that CSTO holds over both COAL and CBTO is that the measurement is very positive. There's no worrying about getting a wobbly ass loaded round to sit exactly straight between a narrow set of caliper arms (COAL); or to have the bullet ogive hit the comparator hole in the exact right position (CBTO).

There's one other benefit to CSTO that I didn't mention earlier... the potential for greater accuracy. Say you're shooting a match this weekend and so you load 100 rounds of your favorite load. Because CSTO is such a fast, positive, precise, and repeatable measurement... it lends itself to quickly measuring every round and allowing you to sort them by those measurements. What I'll typically do is to quickly measure six or seven rounds, pick the measurement that most seems in the middle of the set... and then zero the gauge on that number. I then run through the entire batch of cartridges, sorting from -0025, -0020, -0015, -0010, -0005, 0, +0005, +0010, +0015, +0020, + 0025. Each sort group goes into a different column in a couple of loading blocks to keep them straight. Once they're all measured, they go into my cartridge box in sort order, and are shot in that order. Any rounds that fall outside those sort dimensions (very rare) get set aside.

You could do the exact same thing with CBTO, of course. Except that measuring and sorting 100 rounds of ammo with CSTO is literally two or three minutes. CBTO is simply a slower process.

 

[RegionRat]

Is there a seating die or method that is referenced from the CSTO?

It would seem that all of mine are from the base to the seating stem, and the hope is the difference between the seating stem and the ogive are minimal.

Then, it would also seem the transfer from the base to the shoulder, would add in the dispersion spread in the shoulder datum to base length that resulted from the sizing process.

Am I missing some seating method that bypasses the additional dispersion of the shoulder? Or are we saying the dispersion in the case base to shoulder dimension is near zero?

 

[Jager]

That's been my observation, RR, that with good brass... dispersion from the case base to shoulder is very low.

 

[6BRA]

Bullets are a hot mess. The lengths can be off .010 or more. The nose regions vary bullet to bullet. Custom handmade bullets help. Sierra, Hornady, and Berger...not that great. Sorting helps. Groups of bullets sorted, will be easier to achieve a consistent overall length to ogive. Winter time stuff. A good micrometer die will help in your OCD.

 

[drop_point]

Won't CBTO depend on your ability to control consistent shoulder bump? That alone provides merit to CSTO.

 

[RegionRat]

Won't CBTO depend on your ability to control consistent shoulder bump? That alone provides merit to CSTO.

I think we are getting down to that very concept, that is, if we believe that shoulder length controls where the bullet is actually seated when firing, versus the concept of CBTO.

If for example, the ejector pressure and the firing pin strike, are pushing the cartridge well forward in the chamber onto the shoulder, then CSTO tends to sound like it should be more important since then the shoulder controls where that bullet is located when the firing cycle starts.

That is presumptive on my part, but that may be one of the big differences between a virgin brass firing that is very loose in the chamber, versus a fired case that has only been bumped one or two thousandths.

At the same time, since the only bullet seating methods I am aware of tend to work from the base of the case, then this would imply we need to really control shoulder bump in order to avoid adding a big dispersion factor for the shoulders to the one for bullet seating in general.

I haven't studied my shoulder bump dispersion in a long time, but I know it is never zero.

 

[Old Navy]

Think I will stick to CBTO.

 

[cj8vet]

CSTO simply measures from the shoulder of your case to the ogive of your bullet. If you bump your shoulder .002... that will be reflected in the measurement. If you then decide you want to increase your shoulder bump to .004... CSTO will then show that increased distance. And if your handloading process is largely oblivious to shoulder position - say, because you're following the die manufacturer instructions to screw down the die until it touches the shell holder; or because you mostly neck size - CSTO still works fine for measuring something like seating depth.

COAL, CBTO, and CSTO all essentially measure the position of the bullet with respect to the cartridge case. The advantage that CSTO and CBTO share is that they both are making their respective measurements across a generally well-controlled portion of that case/bullet combination. The area from the bullet ogive to the bullet meplat is often the area of greatest variability, the place where that case/bullet combination is sometimes not so well controlled, and that's why we typically see COAL numbers wander all over the place.

The singular benefit that CSTO holds over both COAL and CBTO is that the measurement is very positive. There's no worrying about getting a wobbly ass loaded round to sit exactly straight between a narrow set of caliper arms (COAL); or to have the bullet ogive hit the comparator hole in the exact right position (CBTO).

There's one other benefit to CSTO that I didn't mention earlier... the potential for greater accuracy. Say you're shooting a match this weekend and so you load 100 rounds of your favorite load. Because CSTO is such a fast, positive, precise, and repeatable measurement... it lends itself to quickly measuring every round and allowing you to sort them by those measurements. What I'll typically do is to quickly measure six or seven rounds, pick the measurement that most seems in the middle of the set... and then zero the gauge on that number. I then run through the entire batch of cartridges, sorting from -0025, -0020, -0015, -0010, -0005, 0, +0005, +0010, +0015, +0020, + 0025. Each sort group goes into a different column in a couple of loading blocks to keep them straight. Once they're all measured, they go into my cartridge box in sort order, and are shot in that order. Any rounds that fall outside those sort dimensions (very rare) get set aside.

You could do the exact same thing with CBTO, of course. Except that measuring and sorting 100 rounds of ammo with CSTO is literally two or three minutes. CBTO is simply a slower process.

 

[Straightshooter1]

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.

Totally agree.

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 ogive,

"below the ogive"??? That suggests to me you're referring to the bearing surface???

My Wilson seating die makes contact on the ogive well below the nose/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.

I guess it depends on what one deems more important (e.g. seating depth, bearing surface contact with neck, distance to the lands, distance into the lands, etc.). Yes, sorting BTO or by base to seating stem contact point includes any variances with the boattail and the bearing surface areas. But, that's fine since my goal is to get the base of the bullet to the same place as measured from the base of the case.

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?).

Yes, to do find significant length variances between the seating stem contact point and the bullet meplat. For example, for some of my sorted bullets by seating stem contact point into groups that are within +/- .001, my 168 SMK's have .014" variance in that area. My Berger 140 Hybrids and 153.5 Hybrids has as much as a .009" variance. And interestingly, to me, my Sierra 6.5 142 gr HPBT that are factory tipped only had .004 variance. . . which might explain to a large degree some of the good consistency I get out of them on paper.

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.

Yeah, I recall you mentioning before your pointing the sorting process before. It seems to me that improvement in BC for LR+ shooting makes up for any variances between those 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.

Given that we're talking about various things involved with changing seating depth (e.g. interior case volume, jump distance, barrel timing, bearing surface variation, even bearing surface contact with neck in many instances), I figure things like .003" increments make a difference on paper as we test our loads, then getting the seating depths as consistent as possible looks pretty important. And the only way I can see doing that is to make sure that when I seat a bullet where the seating stem contact point is the same distance from the base of the bullet regardless of the variations in-between for the different regions of the bullet.

BTW, thanks for your comments, I always appreciate them as they often challenge my thinking, which I like, and help me learn more.

 

[Ned Ludd]

"below the ogive"??? That suggests to me you're referring to the bearing surface???

That should have been "on the bullet nose just below the meplat" - thanks for catching that, I corrected it. I use VLD stems in most of my dies, they appear to contact the bullet nose/ogive about a quarter or three eighths inch or so below the meplat.

 

[mikecr]

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

I seriously doubt that we do..

I believe the major results caused by seating adjustments is not tuning at all.
And I believe optimum seating is a prerequisite to proper tuning, same as finding/using the optimum primer is prerequisite to proper tuning.

You can find a barrel tune and a powder tune no matter where you're seated. But I doubt you're going to see the best results unless you're at least near optimum seating.
I'm sure most here have seen as I have that you can fully tune a gun, and then dial groups to open & close with seating. Each seating tweaking is only changing MV slightly, but having huge affects to grouping.
And it's not moving grouping directionally, but opening no more consistent than shotgun pattern.
That's not tuning..
You don't completely wreak everything with any ~5fps tuning adjustment.
To prove my point, carefully measure MV at each CBTO throughout your full seating testing. Record accuracy(POI to mark) with each MV. Now put the CBTO back to original and adjust to each seating tested MV with powder. Again record accuracy at each MV.
Does it correlate? NO, there is no connection whatsoever.

Anyway, lastly, I think optimum seating (or not) is a quality attribute of bullet-bore interface.
I picture a lottery ping pong ball hitting the mouth of the tube it's sucked into. Sometimes the ball zips straight in, sometimes it rattles into clearing that interface. That rattle, if consistent, is just consistently crappy. If you could measure it, imagine the lexan chamber below with a blast of air propelling a ping pong ball into a throated barrel. The ball is launched from each initial position, and measure is taken with a very hi-speed camera. Can you picture in your mind how the test might show an optimum launch point?
One that anyone would agree, having watched the videos, is for no particular reason -> BEST

It's just my thinking there, and it seems to pass all tests.
The OP seems confused about CBTO and land relationship. They are independent and different terms.
Also, while seating deeper/shallower does affect tune, this can be countered with neck sizing length (tension adj), or powder (to the kernel).
Coarse optimum seating can be found independent of tune, and should be found before serious tuning efforts. Same with primer swapping, and cases should be taken to dynamic stability w/regard to capacity before any actual tuning. All prerequisites

 

[CharlieNC]

I seriously doubt that we do..

I believe the major results caused by seating adjustments is not tuning at all.
And I believe optimum seating is a prerequisite to proper tuning, same as finding/using the optimum primer is prerequisite to proper tuning.

You can find a barrel tune and a powder tune no matter where you're seated. But I doubt you're going to see the best results unless you're at least near optimum seating.
I'm sure most here have seen as I have that you can fully tune a gun, and then dial groups to open & close with seating. Each seating tweaking is only changing MV slightly, but having huge affects to grouping.
And it's not moving grouping directionally, but opening no more consistent than shotgun pattern.
That's not tuning..
You don't completely wreak everything with any ~5fps tuning adjustment.
To prove my point, carefully measure MV at each CBTO throughout your full seating testing. Record accuracy(POI to mark) with each MV. Now put the CBTO back to original and adjust to each seating tested MV with powder. Again record accuracy at each MV.
Does it correlate? NO, there is no connection whatsoever.

Anyway, lastly, I think optimum seating (or not) is a quality attribute of bullet-bore interface.
I picture a lottery ping pong ball hitting the mouth of the tube it's sucked into. Sometimes the ball zips straight in, sometimes it rattles into clearing that interface. That rattle, if consistent, is just consistently crappy. If you could measure it, imagine the lexan chamber below with a blast of air propelling a ping pong ball into a throated barrel. The ball is launched from each initial position, and measure is taken with a very hi-speed camera. Can you picture in your mind how the test might show an optimum launch point?
One that anyone would agree, having watched the videos, is for no particular reason -> BEST
View attachment 1351841
It's just my thinking there, and it seems to pass all tests.
The OP seems confused about CBTO and land relationship. They are independent and different terms.
Also, while seating deeper/shallower does affect tune, this can be countered with neck sizing length (tension adj), or powder (to the kernel).
Coarse optimum seating can be found independent of tune, and should be found before serious tuning efforts. Same with primer swapping, and cases should be taken to dynamic stability w/regard to capacity before any actual tuning. All prerequisites

Good observations. I still don't know what I believe.