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Precision Reloading: .308 Win Example
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Old 08-24-2009, 02:22 AM
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Default Precision Reloading: .308 Win Example



In another thread, we got to talking about various light-for-caliber (hereafter LfC) .308 Win loads and I mentioned that I was working up some loads for resized pistol bullets weighing from 60 to 90 grains. Egged on by the ever-willing-to-egg-on here at THL, I said I'd start a thread on the matter once I had more to offer than just how to 'squeeze' bullets. While I haven't been to the range yet, (you'll see why soon), I have got a good enough start on the process to start this thread.

Since the .308 Winchester is one of my 'most favoritest' cartridges, I thought I might as well wring every last drop of precision I could from this exercise. Knowing that I'd be facing the slings and arrows of nay-sayers and skeptics (not THL folks, but those that might visit), I decided to exercise all the quantitative 'muscle' I could muster. I want the results to be as unambiguous as I can get them. Meaning... I want to minimize the comments about various sources of variability by eliminating as much of the variability as I can and quantifying the rest as much as I can. Strap in, 'cause it's gonna be 'dense'.

My desire with the LfC bullets is to get 'good energy' (lethal doses - 1000 ft-lbs minimum for white-tailed deer) down range with as little recoil as possible. I spend much of my time with women, young, and inexperienced shooters, and recoil is a very real issue for them. Almost as significant as recoil is "lethality". My opinion on the matter of lethality is that the less experienced the hunter, the more psychologically importantit is for the animal hunted to be killed with one clean shot and not have to be chased. The 'experience' is not as "good" if the animal is found dead even shortly after having been chased than if it had been killed "in its tracks". Hence the search for a .308 Win load that is "lethal" on the pointy end, yet "friendly" on the blunt end.

When reloading, one often starts with bullet selection. In this case the suite of bullets from which I was going to choose was:

.312", 60, Hornady HP/XTP 32010 swaged to .3085"
.312", 60, Speer HP GD 3986 swaged to .3085"
.312, 71, Sierra FMJ FN TM RN 8010 swaged to .3085"
.312, 85, Hornady HP/XTP 32050 swaged to .3085"
.312, 90, Sierra JHC 8030 swaged to .3085
.308, 110, Speer SpirePoint 1855
.308, 130, Speer HP 2005




A comprehensive search for the best load for 7 different bullets would be very labor-intensive. (Turns out, it’s pretty darn labor-intensive for “only” 3 of them.) To narrow the list a bit, I started off with QuickLoad (hereafter QL) to see how fast I could get each of them going, and what their trajectories and delivered energies were like. Without boring you with the details of that exercise, the results were that of the resized pistol bullets, only the 85-grain Hornady HP had the ballistic coefficient to 'reach out' to 150-plus yards. I will be going back to fiddle with the others, but for now, the 85-grainer is "it" for the resized pistol bullets.

At my recommendation, sakorick has been experimenting with the Speer 130 in his custom .308 Win. Turned out, the 125-grain spitzer shot straighter than the 130 HP for him, so he was going to use the 125 for hunting. I know what that 130 Speer HP can do terminally, so I want to develop a load that is laser-accurate. (I have a ‘good enough’ load, but I haven’t really fine tuned it.) So the 130 is in this mix for this exercise.

The third bullet is the 110 Speer Spire Point. It's a good step up from the 85-grainer, and down from the 130-grainer. While not an HP, it will be 'screaming along', and based on my experience with other similar bullets, I expect its terminal performance to be excellent.

These loads are being worked up for use in my Ruger M77 with a 22" barrel so I needed to get precise information about the chamber if I was to get the best estimates from QL. The first order of business was fire-forming a case and getting the exact dimensions for creating a "new" cartridge in QL - the ".308 Win My Ruger". (QL allows the user to create new cartridges based on the exact dimensions of a specific case.)

That done, I needed to precisely measure the chamber in order to select a seating depth/overall length for each different bullet. For long bullets, I prefer 1 caliber seating depth - for short bullets, I use 67% of 1 caliber. In the case of the .308 caliber, 67% is 0.206", and since these are definitely short bullets, all of them were going to be seated 0.206" deep as long as the chamber would allow it. Using the Hornady (originally Stony Point) Chamber All, I measured the chamber length from the bolt face to the lands. In this rifle it is 2.288". Clearly, all the bullets could be seated 0.206" deep.

The special case (I made this one, but they can be purchased in common cartridges, or you send them one of your wildcat cases) next to the Overall Length Gauge.


The gauge with the case screwed on.


The case showing the rod that extends inside the case and positions the bullet against the lands when the whole contraption is inserted into the chamber.


After seating the bullet against the lands, the gauge is removed from the chamber and the length is measured from the head of the cartridge to the where the ogive of the bullet hits the lands. In this case, 2.288". You should measure this distance for each bullet you use, OR measure the ogival length of each of your bullets.


Armed with that information, I could get pretty good ‘numbers’ from QL for each bullet. However…

As those of you that have been reloading for some time probably know, “serious” reloaders weigh each batch of cases they get and sort them by weight. While it is rarely, if ever, explained WHY they do this, the implication is that those cases that have similar weights will have similar case capacities. Previously, when I have casually looked at this, I have not found the relationship between case weight and case capacity to be predictable. 'Til now, I haven’t performed a comprehensive examination. Now I have, and weighing cases in hopes of getting cases of similar capacity is an exercise in futility. (Here is one place I expect those ‘slings and arrows’ of which I spoke above to start flying.)

I used a sample size of 54 cases. There were cases from three “lots”. Two were military surplus, (White City Cartridge 1984 and Lake City 1993), and one was “civilian” (Federal Cartridge .308 Win HE).


I have a scale capable of weighing to 0.001 grams (0.01534 grains). To give you some idea of this level of precision, one granule of I3031 weighs approximately 0.003 grams. So I can weigh with a precision greater than one granule of powder. (That's granule not grain.)

To do this “right”, each case had to be prepared identically. First, each case was full-length resized in a Lee resizing die. Then each case was trimmed to 2.010” exactly. Because I would be filling each case with water, each case was “primed” with a spent primer. The primer was inserted ‘backwards’ so that what ever variability between the spent primers was, it was not an issue when inverted. The inverted primer sealed the case uniformly regardless of what type of primer was used.

The procedure was:
Weigh the empty case and write the weight on the case.
Tare the scale.
Fill the case with water using a pipette.
Wick off the meniscus so that the water was exactly even with the mouth.
Empty the case of water.
Write the weight of water (case capacity in cubic centimeters (cc)) on the case.

Here's a picture of what the meniscus looks like.


Here it is after wicking off hte meniscus. This may look concave, but that's an illusion. I assure you the level of water is absolutely flat across the mouth.



Once the weight and capacity in cc was recorded for every case, I entered all of the data in Excel and performed a simple linear regression. The first regression was with all the cases pooled. Then I regressed case weight on case capacity for each separate case group – WCC, LC, and HE. The results are unambiguous – there is NO relationship between case weight and case capacity within a given cartridge. Of course there will be at least two rebuttals to the above statement.

First, someone will argue that the sample size is too small. Wrong. Based on internationally accepted statistical standards, a sample size of 54 is sufficient to determine the magnitude of the variability of a sample that demonstrates the variance of this sample.

Second, someone will argue that “good” (read “expensive”) cases from “good” manufacturers would render results that show a relationship between case weight and case volume. While I cannot say with certainty that this is not true, until I see THE DATA that PROVES it, I’m sticking with the results from this random selection of case manufacturers. There are two reasons to do so. First, the process of ‘drawing’ a case requires that a mandrel be used for the inside of the case. Therefore, the most uniform dimension of ANY case should be the volume. As a manufacturer, why spend money on weighing cases to the 0.001 grams, when the volume will be determined by a mandrel? Second, one of the many “givens” of reloading, is that military brass is “thicker” and therefore weighs more, and therefore has a smaller case capacity, and therefore causes higher pressures in non-military firearms. This data says exactly the opposite. The WCC cases are lighter and have higher volumes. Exactly the opposite of what common “wisdom” asserts. Therefore, I am completely unconvinced that “good” cases will have a high correlation between case weight and case capacity.

In conclusion on the case weight/case capacity relationship: Don’t waste your time weighing cases. Cases with similar weights will not yield cases with similar capacities.

Below you can see the graphs in which are plotted the case weights on the horizontal (“x”) axis, and the case volumes on the vertical (“y”) axis.

Here is a graph of all 54 of the cases. As you can see from the legend, the WCC mil-surp cases (green) are the lightest and have the highest volumes. Just the opposite of what gunwriters (ptooey) will tell you.

Note also that there is no way to predict capacity based on case weight.

Here are the Federal HE cases alone:


Here are the Lake City cases alone:


Here are the White City Cartridges cases alone:


It should be clear that there is no relationship between case weight and case capacity.

Once the case capacity for all 54 cases was measured and recorded, I could use QL and find appropriate charges for each bullet. Theoretically, if I adjusted the case capacity every time I calculated a charge that produced a specific chamber pressure for a given bullet, the resulting muzzle velocities and exit timings would be identical. In fact, that was indeed the case.

I chose 56,565 PSI for the upper limit. This figure is the CIP (European) standard for the max chamber pressure for Mauser 98 actions – an action I use frequently. This figure is well below, (about -6%), the SAAMI max of 60,191 PSI for the .308 Win cartridge. At the low end, I chose 52,667 PSI. It is the default value used by QL for the .308 Win cartridge as the “safe” ceiling for the .308 Win cartridge. The third value – 54,616 PSI - was simply half way between 56,565 and 52,667.

Having chosen the pressure boundaries, I could input each case’s capacity and calculate the charge that gave the exact (within about 20 PSI) chamber pressure I was looking for. It was tedious, but it proved what I was expecting. Namely, by altering the charge appropriate to the case capacity, I could get each case to ‘generate’ the same chamber pressure and therefore MV and bullet exit timing. This means that small changes in case capacity compensated for by charge adjustment, yields uniform velocities and exit timings.

From that point it was simply a matter of setting the charge specific to the case capacity to give either 56,565 PSI chamber pressure, 54,616 PSI, or 52,667 PSI. Each case should then theoretically produce the same MV and exit time. Once that was done ‘on paper’ (in QL), I had all the charges I needed to load 6 cartridges each of three charges for three bullets, or a total of 54 cartridges. (6 x 3 x 3 = 54) In the end, this all took about16 hours to accomplish for the 54 cartridges. And this is why I didn’t get to the range today. J

In summary:
I loaded 18 cartridges with the 130-grain HP, and a charge of I3031 to yield chamber pressures of 56,565 PSI (6 cartridges); 54,616 PSI (6 carttidges), and 52,667 PSI (6 cartridges).

I loaded 18 cartridges with the 85-grain HP, and a charge of I4227 to yield chamber pressures of 56,565 PSI (6 cartridges); 54,616 PSI (6 carttidges), and 52,667 PSI (6 cartridges).

I would have loaded 28 cartridges with the 110-grain spitzer, and a charge of I4198 to yield chamber pressures of 56,565 PSI, 54,616 PSI, and 52,667 PSI, but I found out after I did all the paper-whipping that I only had enough I4198 for 2 cartridges. L I’ll have to recalculate for another powder I have on hand.


Here is part of the table I created that has the proper charge for every case to produce the specific chamber pressure.


I used my .308 Win precision bullet seating die for seating every bullet to the exact seating depth of 0.216.


Here are pictures of the RCBS Precision Bullet Seating Die.




Here is a picture of the loaded cartridges:


I don’t have a .308 Win-chambered rifle fitted with a pressure sensor or I would use it and get chamber pressure data to test if the theoretical estimates (of chamber pressure) made by QL are accurate. I will be measuring muzzle velocity though, and the MVs can be used to estimate chamber pressures. In other words, the MVs that QL estimates are based on the theoretical chamber pressures. Therefore, if the spread of the MVs is as small, it would indicate that the chamber pressures are uniform.

The range I use is closed Mondays and Tuesdays so I can't get to the range 'til Wenesday. As soon as I can, I'll give a range report.

I expect questions from everyone, and number QC by drinksgin

Paul
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Last edited by gitano; 10-09-2011 at 11:08 AM.. Reason: 6 vs 13% and readability
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