Reloading: Case weight does NOT accurately reflect case volume...

Reloading: Case weight does NOT accurately reflect case volume...

This is a discussion on Reloading: Case weight does NOT accurately reflect case volume... within the Defensive Rifles & Shotgun Discussion forums, part of the Related Topics category; For sometime now I've been seeing those inexplicable velocity variations. These are of a nature that 4 of 5 shots have an excellent SD (Standard ...

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    Reloading: Case weight does NOT accurately reflect case volume...

    For sometime now I've been seeing those inexplicable velocity variations. These are of a nature that 4 of 5 shots have an excellent SD (Standard Deviation) and ES (Extreme Spread) but the 5th shot, which can occur anywhere in the 5 shot string, can double and even triple the SD and ES. Where does that come from?

    Here are some actual velocities I measured day before yesterday:

    ........velocities............Vavg....ES....SD.
    1- 2389 2379 2383 2385 2390...2385...11.0...4.5
    2- 2441 2465 2453 2443 2432...2446...33.0..12.6
    3- 2470 2468 2459 2470 2468...2467...11.0...4.6


    Notice there are two sets with excellent numbers, at least excellent for an AR-15, well, not that bad for a bolt gun either.

    But set #2 has one of "those" that move the numbers to significantly higher levels. For example, if I eliminate the high, 2465 fps, the ES drops to 21 and the SD drops to 8.6. While still not as good as the other two, that one shot takes the set from very close to my goal of of ES = 20 or less and SD = 10 or less to significantly outside my goal numbers. While that may not seem like much, the difference in 12.6 (all 5 shots) and 8.6 (high eliminated) is a difference of 46%!

    Even though the percentage difference is high, the actual numbers are reasonable. But, my problem is, I load like this:

    I use Lapua brass, Federal 205M primers, the above sets were shot with Varget driving Hornady 75 gr BTHP-M bullets. Charges are measured precisely on a recently calibrated beam balance scale. Each seating depth is measured and corrected as needed to within +/-0.002". Cases are not weight sorted, but then my PRS buddy doesn't weight sort his cases either.

    Given this care in reloading procedure and components, how could there be a difference of 33 fps? For some perspective, my buddy that I shoot with about 5 days a week, shoots PRS competitions - he won one recently against some prominent PRS shooters. He and another competitor were tied for first place and had to do a man-on-man shoot-off. My buddy won by hitting a 2" dinger at 500 yards with the second shot! That requires an accuracy of 0.4 MOA!

    I said that to say this, he loads essentially the same way I do. And when he loaded up some of my cases/bullets on his reloading gear, we got about the same results I do. Yet, he's getting SD's like 5.0 or less and spreads around 11 or less and getting that VERY consistently. OTOH, I'm loading about the same way and getting those inexplicable velocity outliers

    So, in an effort to determine what's causing the unexpected spreads, I took a close look at the components and how variations in their parameters might affect velocity. First the bullets:

    The bullets:
    Two things with a given bullet type that can affect velocity is bullet weight and bullet diameter. Several other parameters could affect accuracy, but not velocity. Since I'm shooting a 75 gr bullet, I looked at how a bullet weight variation could affect velocity. I'm not sure it would be possible to isolate on this one parameter in the field, so I used QuickLoad to get an idea about how much velocity is affected by bullet weight variations. I believe this would at least be as accurate as trying to somehow isolate on bullet weight alone on the range.

    The velocity of a bullet changes about 1 fps/0.1 gr. That says that for a variation in bullet weight of 0.1 gr, the velocity should change by 1 fps. So even if the bullet weight fluctuates by 0.5 gr, that's only 5 fps - a far cry from that 33 fps , and higher, that I've been seeing.

    Have not looked at diameter variations yet. Will update as I do that. I have however, weighed a number of bullets and found them to be within +/-0.2 grs. So I have an idea of what to use for weight variation. But I haven't measured the diameters yet.

    But, what if the seating depth is off a bit?

    Seating depth:
    I ran this through QuickLoad too. QL indicates a sensitivity to seating depth of 5 fps/0.01". That says if I have one bullet seated at 2.250" and another at 2.260" - that's a 0.01" difference, I should see a change in velocity of 5 fps. Well, that's five times as much as bullet weight, but still a long way from that 33 fps and higher variations.

    So, what about case volume variations?

    Case volume:
    You may have heard as I have that case weight reflects case volume. Taint so! Well, it may be for new cases, but for reloads, my data shows case weight does NOT accurately predict case volume. Here's what I did:

    I randomly selected 20 Lapua, twice fired cases and lubed and sized them, leaving the spent primer in the case - so they would hold water. I cleaned the lube off and numbered each case with a permanent marker. I placed them in order in a case holder and weighed each case. I entered each case weight in a spreadsheet.

    Then, one at a time, I tediously, meticulously filled a case with water using a syringe and observed it under a magnifier until there was a small dome of water. I made sure there was no water on the outside of the case, especially not hiding in the rim area. Then I weighed the case and entered the total weight into another column of the spreadsheet and the adjacent column subtracted the empty case weight from the filled case weight to get the water volume in grains. Then on to the next case.

    Here's the data:



    Us humans, and most of us are, don't process tables well, so here's a picture, or graph of the data:



    The graph is busier than I like, but with a few words of explanation it's pretty easy to understand.

    • First, on the legends, those "poly" and "linear" are trend lines - they really don't need to be on there but I wanted them so...
    • The next extra is the gray dashed line - that's the average case weight line.
    • You may notice the case weight line increases from left to right, yet I said earlier that I randomly selected the cases, so they should not be on a increasing trend from left to right. After I had the case weights, I sorted them from lightest to heaviest because the random order was not easy to interpret.


    At this point, the cases go into a convection dryer at 250 for 40 minutes to be sure all the test water is out. While that's going on...

    Now it's a matter of comparing case weights and case volumes. What we should expect to see is as the case weights go up, the case volume goes down. Remember the cases were sized, so they all have the same external dimensions. So weight variations have to be inside the case. That means that any additional weight a case may have, has to be take up internal space.

    But, if you study the relationship from the chart between the weight and volume, you'll see that some cases follow the expected inverse relationship, but others do not. For example, the first case, #1 indicates a light case and a high volume - perfect! But then the next case, #2 indicates the case is a bit heavier than #1, but it's volume is a lot more than #1 - that's backwards to what we would expect.

    And if you look, the case weight to volume is essentially irregular or inconsistent.

    So what this tells us is that case weight does NOT accurately reflect case volume, at least not for fired cases. And it may be that if these cases had been pin tumbled instead of ultrasonic cleaned the results might be different.

    Well that's interesting, but the question is, how does all this case stuff affect the velocity of a bullet?

    Case volume sensitivity:
    Again, using QL, I find that the case volume affects velocity by 5.3 fps/0.1 gr, where that 0.1 gr is volume determined by water volume in grains. Sooo...

    Looking at the ES of the case volume (bottom of the data table) we see the case volume varies by 1.8 gr. Oops, a correction here, I had given the variation of case weight, 1.8 gr, instead of the case volume (not shown). For these 20 cases, case volume varies by 0.6 gr. Using QL with a case volume of 30.3 gr gives a velocity of 2514 fps and then a case volume different by 0.6 gr, 30.9 gr, gives a velocity of 2482 fps Having that correction in place the change in velocity is 32 fps. That would account for the velocity variation of about 33 fps I've been seeing.

    One thing I did not take into account is overall case length. I did not because I believe, as long as it is under 1.760", case length variations are insignificant. The cases I used are nominally 1.750" +/- 0.002". While it is true that length variation would change the volume of the case, the change is so small in comparison to the overall case it is not significant. For example 0.002" compared to 1.750" is a percent difference of 0.11%, that is about one-tenth of one percent. Plus, volume is linear with length and the variance of length occurs after seating so the larger portion of the case is set, and the much smaller neck area is the slight variance. Since volume is proportional to the square of the radius, the smaller neck starts out with a much lower volume than the larger part of the case. So we have a 0.11% change in length in a portion of the case that's much smaller than the rest of the case - the effect of the variation is insignificant.

    So, next I'll load these cases with the exact same charge and measure the velocity. Hopefully, this will show that cases with a given charge and higher volumes produce slower velocities and vice versa.

    I think there are two conclusions we can draw from this. One, case weight does NOT accurately predict case volume FOR RELOADED CASES. And two, case volume has far more affect on velocity than bullet weight or seating depth.

    Unfortunately, that latter item is a real pain because we cannot reduce the case volume effect on velocity by measuring case weight, and measuring case volume is a real pain!
    OldVet, Havok, G26Raven and 4 others like this.
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    Update on bullet diameter and base-to-ogive dimension...

    I weighed a number of Hornady 75 gr BTHP-M bullets and found that all measured 75.1 or 75.2 gr with one exception that measured 75.3 gr.

    Based on the sensitivity to bullet weight discussed in the above post, a +/-0.1 gr variation in bullet weight is essentially negligible.

    Next I picked out 20 bullets that weighed exactly 75.2 gr and measured the diameter of 10 of them. None of the 10 varied so much as even 0.0005". The all measured 0.2240". After 10 measured exactly the same, I stopped measuring.

    Next was bullet base to ogive measurement. I used a digital caliper and Hornady ogive adaptors for this measurement. Two of the ten I measured were 0.001" longer than the other 8 that measured exactly the same. I lost track of the long ones, but the very slight difference may have been a slight misalignment of the bullet.

    After those incredible results - I quit measuring, what's the point?
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    Senior Member Array RedSafety's Avatar
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    Without reproducing your data and doing a better analysis, I'd say you need to stick to 99.2-99.6 grains to ensure a reasonably consistent volume. At the extremes I would guess wall thickness is an issue. Below 99.2 you may be running thinned, stretched and shortened, walls. Above 99.8 its running thick.

    Rather than velocity, I would concentrate on your groupings rather than velocity. I've seen a few of these multi-variate studies that had some larger velocity variance with lower grouping sizes. If test2 produced the best grouping, go from there.
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    Lapua makes good brass, but because it has thicker walls in the shoulder area, to make my Mongoose cases they would require neck turning. It won't give the required neck-to-chamber clearance needed. LC brass does. Other brands need to be checked before loading live loads. Nosler brass comes very close to needing turning.
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    Quote Originally Posted by RedSafety View Post
    Without reproducing your data and doing a better analysis, I'd say you need to stick to 99.2-99.6 grains to ensure a reasonably consistent volume.
    99.2 - 99.5 is the best match, but even in that range, the data does not show a significant correlation and as we've seen that 0.4 gr volume difference could have a significant affect on velocity.

    Quote Originally Posted by RedSafety View Post
    ...At the extremes I would guess wall thickness is an issue. Below 99.2 you may be running thinned, stretched and shortened, walls. Above 99.8 its running thick.
    The data just doesn't support that. For example, #3 shows for a case wt of 98.8 gr and a volume of 30.5. #15 shows a case wt of 99.8 and a case volume of 30.5. The very same volumes with a full gr difference in case weight.

    This shows that case weight does not accurately indicate case volume.

    Quote Originally Posted by RedSafety View Post
    ...Rather than velocity, I would concentrate on your groupings rather than velocity. I've seen a few of these multi-variate studies that had some larger velocity variance with lower grouping sizes. If test2 produced the best grouping, go from there.
    It depends on what we're trying to do. Velocity variations at short ranges have very little impact on group sizes or accuracy. At longer ranges, velocity variations cause significant vertical dispersion. So for short ranges, i.e. under 400 yards, one can load for accuracy. For longer ranges we need consistent velocities.

    Plus, right now I'm not really interested in accuracy, I'm interested in what it's going to take to tighten the velocity variations. Just like my buddy that shoots PRS gets SDs of 5 and ESs of 10 or less, I should be able to do that too. He shot about a 1/4 MOA group at 100 today and nothing over 1/2 MOA.

    You can have both, velocity consistency and accuracy. I've just got to figure out why I'm getting higher velocity spreads than I want.
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    VIP Member Array G26Raven's Avatar
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    Tangle, I am impressed with your dedication to precision!
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    VIP Member Array hogdaddy's Avatar
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    Case thickness at the rim & even rim thickness could show weight difference & some volume. Just a thought ; )
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    Quote Originally Posted by G26Raven View Post
    Tangle, I am impressed with your dedication to precision!
    Thanks for that!

    I've been able to get some terrific groups, especially lately and even with very wide, well wide to me, velocity spreads. However, given the preciseness of my reloading procedure, it is baffling and frustrating to see those 30+ fps velocity variations. Especially with the guy beside me doing the same thing and getting much better consistencies. The are scattered, here and there, but frequent enough to be significant. It's kinda gotten personal now!

    It may be a combination of things, i.e. the AR type gun, and even the caliber. The .223 case is relatively small and hence small things that affect volume etc. can have a significant impact. Calibers with larger capacity cases are more forgiving of similar variations.
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    A few years back I was experimenting on ways to make inexpensive varmint bullets that are lead free. The easiest to create were a swaged brass case filled with a weighed mix of brass machine dust [ waste from cutting key's] and lead free solder. Once they were properly filled I'd swage the nose into a nice point and then bake them at a high enough temp to fuse the core material to the casing. After just a little fuss I was able to make bullets that weighed pretty close together. What I couldn't do was make them accurate. I weighed them, I sized them, I tried everything I could think of.
    The one thing I couldn't do was control the distribution of the fill inside the bullet!

    What made me think of this is the least accurate bullets were also considerably slower than the others. Maybe wobble eats speed? Just a thought. DR

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    Tangle, I wonder what you'd see with virgin brass that has not seen plastic flow and cold work before. Out of curiosity do you do full length resizing?
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    Quote Originally Posted by HotBrass45 View Post
    Tangle, I wonder what you'd see with virgin brass that has not seen plastic flow and cold work before. Out of curiosity do you do full length resizing?
    Interesting! At the moment I only have new Sig brass; all my Lapua has been reloaded several times. But the Sig brass is pretty impressive - I may give it a try and see what happens.

    Also, I'm not so sure that .223 Lapua brass gets the same treatment as other calibers. It seems "long range" is all the rage these days and I can't help but wonder if the long range brass gets all the attention in manufacturing and maybe the lowly 223 has become less interesting

    I do full length sizing but with a Hornady Match sizing die with a bushing. I tried the Hornady Custom and an RCBS small base (is that the name???) and they crunch the case way too much.

    But the Match die is a full length die.
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    Tangle, do you fire form & neck size after ??
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    Quote Originally Posted by hogdaddy View Post
    Tangle, do you fire form & neck size after ??
    H/D
    Well, kinda, but I'm shooting an AR and they tend to lockup if the brass isn't full sized. But, by kinda, I measure fired cases from base to shoulder either with an RCBS case micrometer or a caliper with a Hornady case adaptor. I size the fired brass for a shoulder 0.004" shorter than the fired brass. That ensures reliability and minimizes working the brass.

    The neck is sized with a 0.244 bushing and then expanded with a Hornady elliptical expander set at 0.221".
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    Senior Member Array RedSafety's Avatar
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    "The data just doesn't support that. For example, #3 shows for a case wt of 98.8 gr and a volume of 30.5. #15 shows a case wt of 99.8 and a case volume of 30.5. The very same volumes with a full gr difference in case weight.

    This shows that case weight does not accurately indicate case volume."

    Handpicking data is garbage analysis. 2 handpicked samples on a decent sized sample size can not be used to make your case, especially when one is an outlier. As I said, without spending time to manually type in your data again and run some decent statistics on it, the data DOES show (eyeballing it) that there are some correlations at both extremes of your case weight data that shows they might be an issue. Some of your variances could possibly be due to procedural error in filling the cases. I'm not certain your methods are that precise for measuring volume.
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    Quote Originally Posted by RedSafety View Post
    "The data just doesn't support that. For example, #3 shows for a case wt of 98.8 gr and a volume of 30.5. #15 shows a case wt of 99.8 and a case volume of 30.5. The very same volumes with a full gr difference in case weight.

    This shows that case weight does not accurately indicate case volume."

    Handpicking data is garbage analysis. 2 handpicked samples on a decent sized sample size can not be used to make your case, especially when one is an outlier.
    You need to eyeball the data again - I'm not hand picking, I'm simply showing results that are indicative of the data. But to show the lack of correlation lets start from the beginning and see what we see.

    Here are the ones that indicate case wt does not correlate to case volume
    #1 98.3 - 30.6 reference point
    #2 98.4 - 30.8 case wt goes up from #1 and so does the volume - that's backwards
    #5 99.2 - 30.6 case wt is 0.9 gr heavier than #1 but the volume is the same

    #6 99.2 - 30.3 - another reference point
    #7 99.3 - 30.4 case wt goes up, volume goes up- another opposite
    #9 99.4 - 30.4 case wt goes up again but the volume doesn't change

    #11 99.4 30.3
    #12 99.5 - 30.3 case wt goes up, volume doesn't change
    #13 99.6 - 30.5 case wt goes up more, volume goes up - again opposite effect

    #14 99.6 - 30.4
    #15 99.8 - 30.5 again, wt goes up, volume goes down - another opposite effect

    This data came from randomly picked cases. These are cases that I'd be using to load with. The data shows that case weight does not accurately reflect case volume over the range of cases I would be reloading with. These are the cases I use. With so many exceptions to the expected effect, there's no way we can claim case wt accurately predicts case volume.

    Additionally the correlation is -0.66 - not very good. 1 or -1 would be exact correlation, 0 would mean no correlation. So -0.66 does not indicate case weight is a reliable indicator of case volume.

    While we could pick a range that has better correlation, what would we do with all those cases outside the range? Never use them? It has been commonly thought that cases can be sorted by weight to get more consistent results. So far, based on this data, it would be pretty much hit or miss on the relationship between weight and volume.

    Quote Originally Posted by RedSafety View Post
    ...As I said, without spending time to manually type in your data again and run some decent statistics on it, the data DOES show (eyeballing it) that there are some correlations at both extremes of your case weight data that shows they might be an issue. Some of your variances could possibly be due to procedural error in filling the cases. I'm not certain your methods are that precise for measuring volume.
    Decent statistics? What statistics would that be? I've included the average case wt, max case wt, min case wt, and ES spread. I did not include the SD, because I'm more interested in what's causing the extremes than the SD. I also have the same stats for the volumes but I didn't include them so here they are:

    Case volume stats
    avg 30.4
    max 30.8
    min 30.2
    ES 0.6

    But, how about I send you a spreadsheet with the data already on it and you can run some decent statistics on it and get back to us?

    As for my methods, I stated my methods in the OP - you must have missed it - I'll post it here again for reference,

    "I use Lapua brass, Federal 205M primers, the above sets were shot with Varget driving Hornady 75 gr BTHP-M bullets. Charges are measured precisely on a recently calibrated beam balance scale. Each seating depth is measured and corrected as needed to within +/-0.002. Cases are not weight sorted, but then my PRS buddy doesn't weight sort his cases either."
    We don't have a gun problem in the US, We have a people problem.
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    in Gun Free Zones.

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