Stupid COAL and CBTO question.

[goodgorilla]

I have not reloaded very much, how constant is the CBTO after seating multiple bullets? I'm guessing the CBTO is constant and the COAL is variable. I guess my real question is does a die seat a bullet to CBTO or COAL?

 

[gstaylorg]

A die doesn't really seat to either one, although the idea is to set the seating depth of the die so that CBTO is consistent from round to round when measured using the calipers/comparator.

The seating stem in a die pushes around the side of the bullet nose slightly below the tip (meplat). Seating die stems can vary depending on the type bullet for which they're designed. For example, a "standard" seater stem has a somewhat cone-shaped recessed hole. Only the outer edges of the recess should touch the bullet slightly below the meplat. However, VLD bullets can have very long pointed noses such that the meplat can actually touch the bottom of the cone-shaped recess before the outer edges contact the bullet further down from the meplat. This is undesirable as it can damage the meplat when seating and often leads to the bullet not being seated straight (runout). A VLD stem has a much deeper recess to accommodate the longer nose and prevent these issues.

Variation in bullet length between the point where the seater stem pushes against it and the point where the comparator gauge sits down on the ogive can lead to variance in the the seating depth. The seating stem should always reach the same point when the ram is extended fully; however, if the bullets vary in length between where you're measuring and where the stem is pushing, the ogive will be farther in/out of the case neck. Sorting bullets by either length or ogive radius can largely solve this issue.

One point to remember is that CBTO measurements really don't mean much between different reloaders. They're completely dependent on the size of the hole in the comparator gauge insert, which can vary greatly between different manufacturers. Even comparators from the same manufacturer can show variation such that two people measuring the same cartridge with their respective comparators will get two different values. In general, some inserts like those from Sinclair seat much closer on the bullet to the ogive/bearing surface junction (slightly larger hole), whereas the Hornady comparator inserts have a slightly smaller hole and seat just a tiny bit farther out toward the nose. However, in this game even "just a tiny bit" may mean .010", .020", or even more, which can make a huge difference in terms of the final product.

Because bullet length within a single lot, or between different lots can also vary quite a bit, COAL measurements are also of somewhat limited use. It's mainly important where magazine length is a critical factor, such that no cartridge over a certain length will fit in the mag. Just be aware that if there is significant variation in bullet length, trying to keep rounds very close to a specific COAL will almost guarantee large variation in seating depth. Much better to seat to consistent CBTO such that COAL doesn't exceed mag length, even though there may be some variation in COAL due to bullet length.

As with CBTO measurements, unless someone is using exactly the same lot of bullets as you, COAL measurements can vary quite a bit, so often you will see a maximum COAL value reported. That way, the cartridges will all fit in a mag, even if the COAL of some rounds within a lot may vary due to bullet length. Generally, due to the potential discrepancies in measurement between different reloaders, I find the most useful measurement description to be a seating depth such as .020" off the lands (i.e. "jump"), or touching plus .010" (jam). With that type of description anyone should be able to reproduce a correct seating depth, regardless of what measuring tools they use or how their chamber specs compare.

I think you will find as you begin the reloading process when your rifle arrives that most of these terms and measurements will become more clear in the doing. Good luck.

 

[Splat]

Wow. Great discription gstaylorg!

 

[goodgorilla]

A die doesn't really seat to either one, although the idea is to set the seating depth of the die so that CBTO is consistent from round to round when measured using the calipers/comparator.

How often do you measure the CBTO after seating bullet?

Sorting bullets by either length or ogive radius can largely solve this issue.

How do you measure ogive radius?

Much better to seat to consistent CBTO such that COAL doesn't exceed mag length, even though there may be some variation in COAL due to bullet length.

I was thinking that, but do not recall reading about it.

Also, when does bullet weight variation come into play? Do you not have to pay attention to it because the CBTO is that important?

 

[Boxcar]

How often do you measure the CBTO after seating bullet?

For the first few rounds after getting your dies into final adjustment. Or every round if you'd feel more comfortable.

How do you measure ogive radius?
With something like the Sinclair bullet sorting stand. Or a similar variance made by a few mfg's.


I was thinking that, but do not recall reading about it.

Also, when does bullet weight variation come into play? Do you not have to pay attention to it because the CBTO is that important?
Weight will always be a factor. If your ringing steel/tactical type shooting I wouldn't worry about it too much. If your shooting F-class/benchrest sort bullets by .10th a grain.

Answers in bold.

 

[goodgorilla]

Do you sort by weight before you sort by ogive radius? I doubt I will be shooting f-class, but I am just curious. Also, that bullet sorting stand looks pretty neat, I'll probably buy something like it.

One crazy question is do you need a zero for each sorted stack of bullets?

 

[Boxcar]

I would sort by Ogive first, your more likely encounter bullets of the same weight. Not knowing exactly what bullets your going to use. I use berger match target VLD and Lapua scenar L's and I experiance almost no variations in bullet weight. But I'm not using a .00 scale, I'm just using a .0. If I used a more expensive jeweler's type scale I would probably get a higher differential.

 

[gstaylorg]

How often do you measure the CBTO after seating bullet?


I measure every one. I measure them using Mitutoyo calipers with a Sinclair comparator tool, except that it has the Hornady inserts (slightly smaller hole). I'm using a Rockchucker press and Redding Type S Match dies, so nothing fancy. I usually do not see more than +/- .0005" seating depth variation with Berger straight bullets out of the box. However, if one ever does come up a couple thousandths or so long, dial down the seating die micrometer appropriately and seat it again. If one comes up more than .0015" or so short, it becomes a sighter/fouler round. Until recently, I did not sort bullets in any way...just loaded them up straight from the box.

However, I have been playing with meplat trimming/bullet pointing recently. I found that I needed a different pointing die insert to get optimal results from trimmed meplats. With the one I had, trimmed meplats looked worse after pointing than they did before trimming. While waiting for the new pointing die insert, I decided to point a few straight out of the box. Once the die was set correctly, they looked great and performed even better. As a result, I started measuring bullets and determined the OAL variation to be about .012"-0.15", for several different types of Berger Target bullets. IMO, this is pretty typical variance. Anyhow, the important point is that if you're not trimming bullets to equal length before pointing them, you have to length sort bullets and adjust the pointing die micrometer appropriately or else shorter bullets will be pointed correctly and the longer ones will have a bulge behind the meplat, or else the longer bullets will be pointed correctly and the shorter ones not pointed enough (or at all). Frankly, I find length sorting bullets much easier and faster than trimming, so that's what I've been doing.



How do you measure ogive radius?


I don't sort by ogive, although I know quite a few F-Class shooters that do. Most that I know personally prefer the tool from Bob Green:

Bob Green New Products




Also, when does bullet weight variation come into play? Do you not have to pay attention to it because the CBTO is that important?



During my bullet pointing/length sorting adventures I described above, I found that sorting several boxes of bullets into 4 groups that varied by ~.003" per group allowed me to reset the pointing die mic by .003" for each group, and that this was a sufficiently small increment that the pointing die could do its job correctly. On a whim, I decided to weigh a bunch of bullets from each length group on my analytical balance, thinking that the longer groups would average slightly heavier. That turned out to be wrong. Just to briefly summarize, the avg weights in all 4 length groups varied less than 0.1%, whereas length variation was slightly over 1.0%, or more than ten-fold higher. To be fair, this was for a few boxes from a single lot of Berger bullets, so other lots and/or manufacturers may be quite different. But the take home message is that I don't sort bullets by weight. Nor do I sort brass cases by weight; it has been demonstrated by many people that case weight does not correlate with volume in a statistically meaningful way. If you want to sort cases by volume, then sort them by volume.

When you begin rolling your own, I think if you do the following, you will be happy with the results:
1) start with high quality virgin brass and bullets from a reputable manufacturer
2) choose a powder that is appropriate for your rifle/bullet/purpose, and weigh it as accurately/precisely as it is possible for you to do
3) become familiar with your equipment so that your neck tension and seating depths are as consistent as possible, with minimal runout

Just make sure you get the basics down first, then it becomes much easier to add various sorting and other fine tuning steps to your routine later. Early on, they can interfere with the learning process. I would also strongly recommend that you follow Dan Newberry's OCW process for load development to start. Follow it exactly, without skipping any steps. It definitely works and will help you get a better feel for things like CBTO, COAL, etc., and how they fit into the grand scheme of things.

Good luck!

 

[goodgorilla]

Good luck!

I heard about trimming meplats can leave them looking worse than they were before. Also, I've heard that if you use high quality bullets like bergers, you don't really need to worry about the ogive variance. Unless your doing fclass or bench rest like you said. I'm interested in this, but how much difference can you make after using high quality bullets?

 

[gstaylorg]

Pointed out of the box has worked well so far for me, although the sample/test size small as yet. The theory is an effective increase in BC from ~3-5%. What I have observed is noticeably tighter groups (less vertical), even at distances as close as 300 yd.

The most convincing (to me anyhow) test I have done so far, I recently used pointed bullets in the first match of the day, then unpointed in the next two. These were shot at a local competition called a "Reduced Palma Match)" that consists of 3 x 15 shots per match at 300 yd. The pointed bullets required between 1 and 2 clicks less elevation (0.125 - 0.250 MOA), again even at only 300 yd. Under comparable wind conditions, I shot a 149-5X using pointed bullets in the first match, 145-4X and 145-5X using unpointed bullets in the 2nd and 3rd matches. Combined with other tests I have done with smaller total numbers of shots, I'm convinced that pointing does give a slight improvement in BC and decreases vertical. At 300 yd, the effect is pretty small. At 1000 yd, it should definitely be more noticeable.

We have two 1000 yd competitions coming up the 1st weekend of Aug at Camp Pendleton and I intend to use pointed bullets in them, so that should provide a little more definitive evidence on whether I'm really seeing what I think I'm seeing at shorter range. However, as I mentioned before, I think you're better off not trying to incorporate all the little sorting, pointing, etc., reloading tricks that people sometimes use right out of the gate. Just my opinion, but there is a lot to learn and become proficient with just doing the basics. Trying to do all the little "extras" at the beginning just adds a whole new level of complication to the process. Further, I have had very good success with the basic loads I developed without using doing any of the extras, so it's not like you can't obtain a very good load without them. Once you have a good solid load, you can add easily the other refinements at any time without changing the load specifics.