Bearing surface: body length vs. base-to-ogive

[edlongrange]

From Lynn:

"Monte:

It helps eliminate fliers. If your shooting VLD's you are already seating to 0.001 variation or your losing matches. It does the same thing.

Lynn"

 

[memilanuk]

lynn (via ed),

Not shooting VLDs but generally speaking, yes, I strive to keep my seating depths consistent to w/i a thou. Been fussing a bit with case necks lately in pursuit of same, but that is the goal.

Thanks,

Monte

 

[PrecisionRifleBlog]

Here is a diagram that illustrates the difference between the base to ogive measurement and true bearing surface.

I wrote an article that has a brief explanation of bearing surface, how to measure true bearing surface, and the bearing surface variations found in match bullets. I thought you guys might find it helpful.

Check it out at: http://precisionrifleblog.com/2013/01/29/berger-bullets-bearing-surface-variation/

 

[memilanuk]

Hi-jacking an old thread for self-promotion... nice... ???

 

[Steve Blair]

Hi-jacking an old thread for self-promotion... nice... ???

[br]
"Well, I guess hard times flush the chumps. Everybody's lookin' for answers."
Ulysses Everett McGill [br]
:

 

[Nomad47]

Actually, aren't there two bearing surface lengths? One where the bullet contacts the lands - and another where it contacts the grooves.
Also, when the bullet engraves, that portion of the bullet contacting the lands has to go somewhere - probably altering the length of the bearing surface that contacts the grooves. ;D

 

[jlow]

This is something that I’ve always wonder about, which is would variation in a bullet’s base-to-ogive length affects case volume after the round is put together?

To back off a bit, this comes back to the question of what is one doing when one adjust the seating depth of the bullet which is a common technique to tighten up groups. One effect of course is to change the distance of the bullet (bearing surface) to the lands. One can certainly see why this might be important if you are seating close to the lands, but interestingly changing seating depth also has a huge effect on rounds where the bullet is seated close to mag length where of course the bearing surface is a long distance from the lands. So this begs the question of another possible effect i.e. changing the internal volume of the case when the bullet is seated deeper. We know for instance when we use a case with thicker walls (i.e. LC 308 brass or Lapua 308 brass for that matter) that you have to adjust powder charge if say you switch from a thinner wall Win 308 case. So why would seating a bullet deeper and reducing case volume not affect things like pressure etc. ?

So going back to variation in base-to-ogive length. I realize that the comparator is not using the same dictum point as a seater but assuming that one reflect the other, would not a longer base-to-ogive length bullet be seated deeper into the case and have the above effects? The lengths we are talking about are not huge ~ 1-15 thousands if you are talking about say Sierra 308 175s, but of course here now we are dealing with the volume of the bullet associated with the length in question.

 

[jlow]

To address the question of relationship between bearing surface vs. base to ogive measurements, here are some data I collected today sorting through 500 175gr SMKs. The first photo is my sort box – sorting is complete with the mito caliper configured for bearing surface measurements. The number above each bin is the bearing surface length. For example “497” would equal 0.497 inch in length.

I have picked out 25 bullets with a range of bearing surface length and measured their base to ogive measurements. This data is shown in the Excel spreadsheet in the next screen dump. There is good correlation between the two numbers for the same bullets with a correlation coefficient of 0.799. The average extreme spread and SDEV for bearing surface and base to ogive was the same i.e. 0.02” and 0.0039”, respectively.

So a couple of conclusion from this data.

1) At least in this box, measuring bearing surface length there appears to be one single batch of bullets with a Gaussian like distribution with one single peak i.e. between 0.498-0.499” – I have seen two in one box but apparently not this one.

2) The range of bearing surface length can be significant i.e. 0.02” from longest to shortest. Imagine having the shortest and longest bullet in a single grouping shoot…

3) Using a small 25 sample, there appears to be an excellent correlation between bearing surface length and base to ogive length so one could use one or the other measurement and assume the other is the same.

 

[Steve Blair]

2) The range of bearing surface length can be significant i.e. 0.02” from longest to shortest. Imagine having the shortest and longest bullet in a single grouping shoot…

[br]
Why imagine? Try testing your hypothesis. Shoot groups with widely varying and identical measurements and see what the difference is. Personally, I don't think it will make much difference. My testing shows seating depth variation is far more significant and this type of sorting does not address that problem.

 

[jlow]

I think you missed my point. If you take two bullets, one which is 0.02” shorter in base to ogive and you seat them to the same OAL, the one bullet with the shorter base to ogive will have a case volume equivalent to seating the other bullet 0.02” longer. So if you believe seating depth affect bullet grouping, then you should believe variation in base to ogive is an important parameter.

 

[Steve Blair]

I think you missed my point. If you take two bullets, one which is 0.02” shorter in base to ogive and you seat them to the same OAL, the one bullet with the shorter base to ogive will have a case volume equivalent to seating the other bullet 0.02” longer. So if you believe seating depth affect bullet grouping, then you should believe variation in base to ogive is an important parameter.

[br]
Incorrect. The powder volume variation you are discussing is insignificant. A few thousandths seating depth variation can make a large difference. Just because you can measure and graph a parameter does not make it significant. And, correlation does not equal causation. Conduct some tests to prove your hypothesis and present the results. Anything else is idle chit-chat. [br]
Additionally, OAL is meaningless. Only the distance from the bullet leade contact datum to rifling leade is relevant.

 

[jlow]

OK, so let’s go through the list.

First, you say powder volume variation is insignificant. Correction, what I was mentioning is case volume which is different than powder volume.

Second, we both agree that seating depth variation can make a large difference but you are saying OAL is meaningless because you think only distance from bullet leade contact datum to rifling lead is relevant. I would disagree with this since bullet seating depth for rounds seated to mag length (which is a long distance from rifling) also makes a big difference. I know this from first hand.

Third, I don’t really think one should be-little people since it is really not conducive to a constructive discussion. The fact is what I have show thus far is at least REAL data. Now I am not saying what you say is all incorrect but at least at face value, it is all talk and no data. If you want us to believe what you say is correct, show us some actual data to prove the point. After all, fair is fair, you say go do some test, present the results or anything else is chit-chat – it works both ways.

 

[Steve Blair]

OK, so let’s go through the list.

First, you say powder volume variation is insignificant. Correction, what I was mentioning is case volume which is different than powder volume.

Second, we both agree that seating depth variation can make a large difference but you are saying OAL is meaningless because you think only distance from bullet leade contact datum to rifling lead is relevant. I would disagree with this since bullet seating depth for rounds seated to mag length (which is a long distance from rifling) also makes a big difference. I know this from first hand.

Third, I don’t really think one should be-little people since it is really not conducive to a constructive discussion. The fact is what I have show thus far is at least REAL data. Now I am not saying what you say is all incorrect but at least at face value, it is all talk and no data. If you want us to believe what you say is correct, show us some actual data to prove the point. After all, fair is fair, you say go do some test, present the results or anything else is chit-chat – it works both ways.

[br]
Fine, case volume. [br]
You did show data, but no results linked to it. So, what's the point? I'm not belittling you, just trying to get you to prove your point. Your graphs are real nice. Lacking results, what can you say they mean? [SIZE=small]If you run some properly structured tests against your hypothesis, you have the opportunity to either prove it or learn something. If you present the results here, we can all learn something. I've done this before and likely will again. You should, too.[/SIZE][SIZE=small] [br][/SIZE]
Monte (the OP) is an F-Class shooter. I seriously doubt that he worries much about magazine length. In any case, it should be obvious that the discussion is about factors affecting accuracy. If you have to load to magazine length, so be it. It just has little to do with the original question. [br]
I sort bullets using Bob Green's tool. It measures from leade contact datum to seating stem contact. By sorting to .001" groups, my seating depth, at the leade datum, is <±.001" from nominal. If I wanted to present a case for using the tool, results directly linked to the varied parameters would be expected. [br]

 

[jlow]

First, I have a Ph.D. degree and come from the scientific world with 30 years of research experience so I think I can speak with some authority about doing research.

I would agree that I have only shown some data but no shooting results. The reality in research is nothing is done in one step. Research start with gathering data, making observation, coming up with hypothesis based on observations and discussion and moving on step by step. What my results mean is not completely known but I have in fact not only given data but some conclusions. Now, those conclusions may or may not be correct but that is in fact the name of the game when it comes to research. Do some studies, come up with conclusions, test your conclusions and put up new conclusions. This is basically what I am doing. If only absolute un-refutable results is what you are looking for, that is not how research is done in real life. Even the best shooters in the world are always coming up with ideas and challenging and testing them, it is never over.

You miss the point about mag length. It is data that tells you the effect of changing seating depth are not just all about how close you are to the rifling lead. Unfortunately, some shooters only live in their narrow world of jamming but if one open their mind and looks outside of jamming, it quickly becomes evident that there are things one can learn about their own reloading technique from other areas even though it does not directly apply to their area of interest. Knowing and understanding is how we make progress.

 

[Steve Blair]

Well, I worked at General Atomics for sixteen years. I worked with many PhD's, both Physics and Engineering, who make fine theoreticians, finite element or thermal analysts, but couldn't design a paper bag to effectively hold their lunch. Much better to cite results than credentials. [br]
I look forward to seeing your experimental results presented here. My testing years ago convinced me that measuring the parameters described do not significantly contribute to accuracy. However, if you present a persuasive case, I would be forced to revisit the subject and attempt to duplicate your results. That is the very point of publishing experimental results, as you know. [br]
Regarding jamming; Since I shoot F-Class and sometimes need to extract a round while assessing conditions, I do not jam. The Berger Hybrids I shoot seem most amenable, in my testing, to jumping from ~.015-.020".

 

[jlow]

The honest truth is it is my believe that base to ogive length makes a difference but do not have any data to proof it.

I happen to be sorting bullets today and simply wonder if my bearing surface length in any way related to base to ogive length. Having found the data I graphed which I thought was interesting, it was my thought that I would share this with the OP with some ideas of what it may mean hoping that someone out there has done the study.

The sad truth (or the good part) is if you look at the binning, the SMK are mostly pretty tight in terms of bearing surface length and if one was serious about testing the hypothesis as to whether it “matters”, you will need significantly more outliers than just a few like what I found. Only robust studies (read multiple, repeatable) with significantly numbers (what we call N-size) is required to proof something like this.

 

[Steve Blair]

The sad truth (or the good part) is if you look at the binning, the SMK are mostly pretty tight in terms of bearing surface length and if one was serious about testing the hypothesis as to whether it “matters”, you will need significantly more outliers than just a few like what I found. Only robust studies (read multiple, repeatable) with significantly numbers (what we call N-size) is required to proof something like this.

[br]
Definitively, yes. But that does not negate the value of a preliminary study, using a smaller sample population, to see whether there is any meaningful variation. I always conduct a "proof of concept" study before expending the time and components necessary for results can be regarded with high confidence.

 

[mikecr]

jlow, I agree with sleepygator all the way here.

If you test & show that bullet/bearing displacement capacity variance on the order of ~10thou typical(or 20thou even) can be significant to a common capacity per cal, that would be interesting. You would be the first to ever do it as far as I know.
But what stands out in the measure you graphed is that you didn't first qualify bullet noses by ogive radius.
This invalidates your measure and weakens my confidence in your methods.

 

[jlow]

Sleepygator – It would be an interesting study and if I have a chance to do it, I will certainly report back.

Mikecr – The way I think about it is if one seats a bullet 20 thousands deeper, it is certainly not within the whelm of possibility that the group tightness can be affected – it has happened to me following an OCW. I think of it this way, the Hornady comparator I use to index on the ogive is in many ways similar to a bullet seater which also index on the ogive. Since there was as much as 20 thousands difference in length at that ogive location relative to the base, then if the same difference in distance can be translated to the area of the ogive used to push the bullet in by a seater, then the base would be in 20 thousands less deep in the case. Now I freely admit that there is an assumption there, but it is not a wild assumption if the difference in the SMK is in fact in the area past the ogive i.e. the bearing surface. This is something that I freely admit I don’t know that that is why I propose the theory here.

As to your question of “qualify bullet noses by ogive radius”. This I think points to what I am talking about in the above paragraph. If you must know, there are a number of things that has stopped me short from doing what you want to know. One is it would require me to take my seater apart to do this and this is something that I would very much like to avoid since it is currently seating to a very specific depth that I using – something I am sure you I hope as a precision reloader would understand. Second, as you know this is a tricky and difficult measurement because it would require an accurate measurement of the inside diameter of a hole, something that I do not know of an accurate way to do. Without an accurate measurement, the question regardless of effort would remain open and unanswered. So at least to me, trying to answer the question this way to me is problematic.

The reality is this cannot invalidate my measurement because my measurement is what they are and they clearly stand i.e. the relationship between bearing surface length and base to ogive measurement relationships. It is a weakness in my related hypothesis but if there was not a weakness then it wouldn’t be a hypothesis but a fact. That is why I put in Reply #27 that “I realize that the comparator is not using the same dictum point as a seater” to point out up front the unknown and in fact assume (without measurement) that they are different. It also cannot weaken your confidence in my methods since the weakness is not in my method but in the hypothesis and that of course is what we want to find out through experiment.

 

[mikecr]

Steve mentioned qualifying ogives with a BGC, you should not dismiss this.
If you would like to test your notions about capacity as affected by seated bearing and base area, then you really must eliminate ogive radius variance, so that you can first establish an amount of bearing/base area seated. Even if only relative.
As it stands, with your current measure, you do not know bearing length, base length, or how much of what will affect capacity.