Effect of bearing surface length/base-to-ogive length on bullet seating depth

[jlow]

To avoid hijacking another thread, this is a continuation of the conversation I have been having in that thread – this one:

http://forum.accurateshooter.com/index.php?topic=3790387.0

In that thread in Reply#28, I measured five hundred 175gr SMKs and found that their bearing surface length (BSL)/base-to-ogive length (BTOL) numbers were similar but varied by a maximum of 0.020”. I hypothesized that this difference in BSL/BTOL might affect the seating depths of the bullets and thus case volume available for power, and possibly chamber pressure/accuracy. So I set out to prove that this difference in BSL/BTOL can in fact translate to a significant difference in seating depth.

Since it is very difficult to determine exactly where the Hornady comparator tool touch the ogive of the bullet vs. where the bullet seater touch the ogive, I came up with the idea that perhaps the best way is to go directly to measure how deep the two bullets with 0.02” difference in BSL/BTOL would seat in a case.

So here is what I did:

1. Chose two bullets from the two extreme in BSL/BTOL.
2. Measured bullet weight, BSL, BTOL, and actual bullet length (base to meplat).
3. Chose two Win 308 cases and measured case weight, and length.
4. Seat the two bullets in the two cases normally and measured OAL (ogive) and OAL (base to meplat) of the completed rounds (minus powder and primer).
5. All measurements were done three times with a Mitutoyo Absolute caliper.

The results can be seen in the attached screen dump from my Excel spreadsheet. So a number of observations.

1. There were slight differences in the weight of the two bullets – 0.0733 gr.
2. BSL/BTOL as expected were significant in the 0.0190 to 0.0202” range.
3. Actual bullet length (base to meplat) difference was less – 0.0028”.
4. Difference in the weight of two cases used was slight (as expected since there are weight sorted cases) - 0.44 gr.
5. Case length was very slight (as expected since these are trimmed cases) – 0.0012”.

The bullets were seated with a Forster Ultra Micrometer Seater die and both OAL (ogive) was very similar 2.2183” vs 2.2185”. This is not a surprise as the seater normally can seat reproducibly within 0.001”

What is interesting is the OAL (base to meplat) was significantly different with the round containing the bullet with the longer BSL/BTOL being about 0.0173” shorter. Keeping in mind that the shorter bullet’s overall bullet length (base to meplat) was 0.0028” shorter, I added this number to the OAL (base to meplat) of the shorter round to normalize its length which brought its length from 1.7993” to 1.8022”. With this new number, the actual length difference between the two rounds is 0.0202” which is exactly the difference in the BSL/BTOL length of the bullet.

So this appears to confirm that in fact a bullet with a longer BSL/BTOL will seat deeper into the case and reduce case volume.

To further confirm the results., here are two photos. The first is of the two bullet/case combo. I put three lines in there showing that the two cases were identical except for the location of the end of their meplat. The right combo has the bullet with the longer BSL/BTOL and the bullet is seated in further.

To make the point even clearer, I took a surgical scalpel and scored the bullet where they were exposed at the neck of the case. The bullets where then pulled with a Hornady Cam-Lock bullet puller and places side by side and this can be seen in the third photo. You can see how much higher the score mark is in the bullet with the longer BSL/BTOL again showing that the bullet was seated deeper into the case.

Using the Photoshop measuring tool and the known fact that the bullet diameter is 0.308”, the difference in the score marks is around 0.02”.

So it would appear that all the numbers confirm that a bullet with 0.02” longer BSL/BTOL will seat 0.02” deeper in the case.

Since the normal SOP for tuning groups after an OCW is to seat bullets deeper by 0.01” increment, it is clear that it is worth sorting bullets by BSL/BTOL if they vary anywhere close to this magnitude.

 

[Barlow]

When I shot the same SMK's, I had to sort them by length ( base to ogive) and seat them as sorted. Barlow

 

[jlow]

According to my data, that my friend is what you are suppose to do. ;D

 

[Raythemanroe]

Can I just say this seems like a lot of work for something that is pretty much common sense. I will follow up with you will get different results with different reloading die's..Some manufacturers seat with the ogive and others off the melplate which I wouldn't recommend..

 

[jlow]

LOL! You can say that and I can even agree with you but if you take a moment to read the comments that I got when I first proposed this idea in the other thread I linked to the OP, you might understand why it was done.

For a “technical board”, this is the thing that is sometimes disappointing to me and that is when someone comes out with an idea, a lot of times the response is less than technical and non-constructive. However, when that same person then goes ahead and do the work to prove the point, the silence is deafening except for something like this which is “lot of work for something that is pretty much common sense”. :-\

 

[Raythemanroe]

I don't disagree with you at all and who ever is must not know why they sort there bullets!

 

[Elkbane]

Sorting will help, but it misses THE most important variable - the shape profile of the nose. Let me explain.

Seating depth depends on two settings, one is controllable and the other is not. When you are seating, you have a fixed diameter hole in the seating stem and you control seating depth by positioning that hole at a given depth from the base of the case by turning the seating stem in or out. The seating stem contacts the bullet on a datum line equal to the seating stem inside diameter.

When you sort bullets, you HOPE that by "batching" bullets with like base to ogive characteristics you will also produce slimlar lengths from base to the point on the nose where the seater stem contacts the bullet, but this isn't always the case. What really matters is having bullets not only with the same base to ogive, but ones with similar taper profile between the ogive and the tip; i.e. the nose. If you want to test this, take your sorted bullets and seat them. Then measure base to ogive on all the seated rounds. The difference you measure is absolutely a function of differences in the shape of the nose between ogive and tip - that is the only point of contact inside the seating die.

IMO, if you have enough variability in the bullets base to ogive to want to sort them, you are going to have large variability in the shape of the nose as well, and that's almost impossible to sort out, unless you use a small diameter (like 17 cal) insert in your tool to do a secondary sort as a proxy for seating stem datum diameter.

A manufacturing process is either producing high variability or not, based on the QC processes in place. If they are bulking bullets across a production line (probable) the end result will have higher variability than if you had only in-line variability.

ELkbane

 

[Raythemanroe]

A Reading type seater works more to the ogive and not the tip, this works better for the vld type bullets..I get where your coming from and think a tool matching your stem or die to measure base to ogive would be beneficial in sorting bullets..Maybe your on to something, better patent it! Maybe design a die that you can measure with the stem ( comes apart) to sort bullets!

 

[jlow]

Elkbane – I think when I sort by base-to-ogive, I do assume that the shape profile is the same and it is the cylindrical bearing surface that is changed. It is indeed a weakness of the method but short of have an optical comparator, that seems to be the best one can do?

I think the data I have shown would disagree with you that the difference in base-to-ogive length is “absolutely a function of differences in the shape of the nose between ogive and tip”. To me, the big surprise of my study was not the fact that the longer base-to-ogive bullet seated deeper, but the fact that the data appears to show that the difference between the long and short bullet was the length of the bearing surface instead of a difference in the shape profile of the nose.

The reason I say this is because it is unlikely that where the Hornady comparator dictum point on the nose is exactly the same as where the seater is pushing. If they are not touching on the same point on the nose, would not the only way that the increase in seating depth is exactly the same as the extra length be if the extra length is all in the bearing surface?

I would be interested in knowing why you think the variability is in the shape of the nose – is this conjecture or do you have scientific evidence to prove this?

Raythemanroe – I would agree that a seater that push down on the melpat is a very bad idea.

 

[REastman]

I would be very curious to see what results you get if you took 5 of each and shot them. Does the .020 seating depth difference actually make any difference in the end result? Is there a notable speed difference over a good chrono, or a change in POI between the two extremes?

 

[jlow]

I could do the study but it would certainly take a lot more than 5 bullets to obtain any results that would be statistically significant – it’s the nature of statistics.

The good news is the effect of seating depth affecting group size is something that is well known and recognized by precision reloaders. For example if you read the well known article here on OCW method of load development by Dan Newberry which a lot of the folks here use

http://optimalchargeweight.embarqspace.com/ocw-instructions/4529817134

If you look at instruction #17, he says:

"17. Remember, don't get "bowled over" by a tiny group which falls outside the OCW zone. You can tune any of the groups to be tiny with bullet seating depth changes. After you have determined the OCW, you may want to try seating the bullets deeper or longer in .010" increments to see where your particular rifle does its best. I have often found that OCW recipes are so reliable that seating depth alterations--especially for hunting cartridges--often don't seem necessary"

 

[Elkbane]

jlow,
Wasn't trying to pick an arguement with you - hope I didn't come across that way. What you're doing is spot on - trying to control variability is what we try to do in reloading. And sorting the way you are doing is is probably the best bet, but I guess what I was saying is that for the difference in prices between bullets, my vote is to buy ones that don't require sorting or ones that are relatively jump intolerant. For my processes, with my equipment, for my purpose (casual, but intensely self-competitive target shooting), that's where I came out on the time/expense/result continuum.

I came to my conclusion two ways - early into reloading I noticed that seating depth to ogive had some variability in it that I couldn't explain, and I was shooting 130 SMK's out of a 7mm-08. So I sorted some bullets to ogive and laid them aside in two 10 round batches. I loaded 10 of them and the seating depth variability was better than unsorted, but not as much as I expected. So I took the other 10 and re-sorted them using a 20 cal insert on my comparator (smallest I had). There was as much variation at 20 cal as there was in the inital sort at 284 cal!!.

Then I let logic take over and realized that the die is for all practical purposes fixed by the ID of the seating stem and the length you have is set ( Sometimes a pause to assess the basic process is helpful). Absent any bullet deformation caused by the seater stem, the length at which the OD of the bullet = the ID of the seater plug determines seating depth. Unless your press flexes..... Whatever variability is in the batch of bullets relative to the shape of the nose will show in the loaded rounds, as the caseholder under the casehead and the seater plug are the ONLY places the the seating die "touches" the loaded round..... If you can reduce that source of variability by sorting to ogive, then it may be worth doing.

FWIW I repeated my test on 123gr Lapua Scenars for in 264 cal and determined, at least for me, they weren't worth sorting.......
Elkbane

 

[Elkbane]

jlow,
Wasn't trying to pick an arguement with you - hope I didn't come across that way. What you're doing is spot on - trying to control variability is what we try to do in reloading. And sorting the way you are doing is is probably the best bet, but I guess what I was saying is that for the difference in prices between bullets, my vote is to buy ones that don't require sorting or ones that are relatively jump intolerant. For my processes, with my equipment, for my purpose (casual, but intensely self-competitive target shooting), that's where I came out on the time/expense/result continuum.

I came to my conclusion two ways - early into reloading I noticed that seating depth to ogive had some variability in it that I couldn't explain, and I was shooting 130 SMK's out of a 7mm-08. So I sorted some bullets to ogive and laid them aside in two 10 round batches. I loaded 10 of them and the seating depth variability was better than unsorted, but not as much as I expected. So I took the other 10 and re-sorted them using a 20 cal insert on my comparator (smallest I had). There was as much variation at 20 cal as there was in the inital sort at 284 cal!!.

Then I let logic take over and realized that the die is for all practical purposes fixed by the ID of the seating stem and the length you have is set ( Sometimes a pause to assess the basic process is helpful). Absent any bullet deformation caused by the seater stem, the length at which the OD of the bullet = the ID of the seater plug determines seating depth. Unless your press flexes..... Whatever variability is in the batch of bullets relative to the shape of the nose will show in the loaded rounds, as the caseholder under the casehead and the seater plug are the ONLY places the the seating die "touches" the loaded round..... If you can reduce that source of variability by sorting to ogive, then it may be worth doing.

FWIW I repeated my test on 123gr Lapua Scenars for in 264 cal and determined, at least for me, they weren't worth sorting.......

What this test really did for me was make me question my basic assumptions on results of changes to seating depth using small batch firing tests. If you are going to test, you better be prepared to shoot enough of them to be satisfied that the result is due to the change and not bullet to bullet variation....and also it confirmed my belief that "just kissing the lands" is the worst place you can be - you may have them +/- 5 thou in or out at random and not know it if there is that much variation in the bullets themselves.
Elkbane

 

[jlow]

Elkbane – Thank you for your interesting story of your experience, quite educational! Having gone down many dead ends myself, I certainly agree with you when it comes to using quality components to streamline the way to perfection. It is really the way that I started which is with less expensive components as I learned reloading but slowly moving to the premium stuff nowadays. However, I do find it is useful to do this type of troubleshooting studies because they teach me a lot as it relates to the parameters which affect precision/accuracy and of course not all of us can only use the best and most expensive stuff.

In terms of my theory, your input is absolutely not a problem and I did not take it negatively. It is good that you have put your views forward, makes me think and in this case made some additional progress. I have in fact done a lot more thinking about this and I actually think that we both may be right. Here is a set of drawings I put together today to study the possibilities and I will use it to show you how I came to my conclusions.

There are four sets of 2 bullets in this drawing – they show some of the possibilities that may represent what we are looking at with the SMKs. The group (Group 1) on the extreme left labeled “Normal” are your normal boat tail bullet. The next three are bullets that have changes in them. The “Deform down in nose” bullet (Group 2) has the nose cone which have swollen in diameter. The “Longer in the nose” bullet (Group 3) has its nose cone pulled out so that it is more slender, and the “Longer on bearing-surface” bullet (Group 4) just has its bearing surface lengthened.

So of the three changed bullets, only Group 3 and 4 bullets has longer OAL as measured from base to melpat. In terms of bearing surface, only Group 2 and 4 bullets have lengthened surfaces.

The lower sets of drawings shows the same four different types of bullets being seated into a case using a seater die which pushed down on the ogive. Line “C” shows that all three bullets have been pushed down by the same seater to the same degree but the combination using Group 2 bullets are shorter in OAL (base to meplat). Combination using bullet from Group 3 has a longer OAL, and combination using bullet from Group 4 has an unchanged OAL from the normal Group 1 bullet. In terms of seating bullets deeper, this happened when using bullets from Group 2, 3, and 4 with 4 being the deepest.

Looking back at the bullet I examined which has the longer bearing surface, it has the same length as the normal bullet, when seated in the case its OAL (base to ogive) was shorter than when the normal bullet was used and it seated deeper than the normal bullet.

So from the three possible examples shown, I think it was a Group 2 bullet. The reason I say we are both right is because the nose cone was in fact deformed as you have guessed. However as it deformed, it widened the area of the nose cone next to the normal bearing-surface area thus in essence lengthening it.