Of Case Capacities, Bulk Densities, and Accurate Predictions

[gitano]

I conducted a search on "bulk density" and looked through the archives. I saw some threads that discussed the issue of "bulk density", (hereafter BD) but I didn't see any that;
1) compared MEASURED BDs and
2) discussed the effect small changes in QuickLOAD'S (QL) "standard" BD values make on getting accurate estimates out.
If I missed them, please provide links so I can read them.

I have what is essentially no interest in "factory" cartridges. I've been reloading cartridges from .17 to .72 caliber for almost 50 years. As an inveterate wildcatter, I find QL both indispensable and invaluable. When I first started using it I got excellent results using the "standard" values. However, over the years as I delved deeper and deeper in to the individual components, my actual values for muzzle velocity (MV) and maximum chamber pressure (MCP) have varied considerably for more and more cartridges. Some blown primer pockets got me looking hard at "standard" BD values and I found that there was often great disparity between the values in QL and my actual measured values. As a result, I made a device to facilitate measuring bulk density. Now, whenever I buy a new canister of powder I measure the BD and record it on the canister. It gets remeasured every 12 months. (As the powder ages, the volatile 'driers' evaporate, and the BD increases.)

After doing 'things' this way for a few years and continuing to get 'anomalous' results, it dawned on me that there was an unnecessary 'conversion' being performed; The conversion from "grains of water" in the QL set-up, to "load density" based on the BD value. The direct way to deal with this is to simply determine actual case capacity for a specific powder by using the specific powder to determine case capacity. Of course this is tedious, and I am neither suggesting or recommending that this be done for "quick and dirty" preliminary calculations, but when you're getting 'strange' data, checking the actual BD of your powder is something to consider. Of course the "grains of water" to "grains of powder" conversion is perfectly fine as long as one has determined the actual BD of the powder(s) being used.

I am most recently involved with making an AR-10 chambered in something akin to the .308 Win necked up to 8mm. Using QL to get starting loads, I foud the QL estimates of average MV as much as 150 f/s in error. QL estimates - after I put in good input data - are usually within 25 f/s of actual. After taking precise measurements of the barrel, examining fired cases, and looking at all the variables, it became clear that the "standard" BD values were sufficiently off to create 'problems'. "No problem" says I, "I'll just adjust the case capacity based on the actual BD of the powder I plan to use." (That's perfectly fine as long as your'e not changing powders as a result of changing bullet weights.) As I moved from 125 grain bullets to 220-grains bullets, the powder of choice (highest MV with lowest MCP and up to but not exceeding 105% of case capacity.) I got some "hot" loads. Hmm... What I realized is that I had set the case capacity based on the BD of a particular powder. This is not 'wrong' because once the correct conversion is made based on using the correct BD, the case capacity SHOULD be "right". The error on my part was assuming that the BD for the next powder was close enough to the "standard" value that the case capacity determined by using powder was correct for "the rest" of the powders. Here are some numbers to illustrate how far off that can lead one to be.

Using Hodgdon's CFE223, the case will hold 64.90 grains of powder. Setting the seating depth to 0.001", (essentially "zero" but eliminating any 'confusion' on QL's part about "seating depth"), putting 64.9 as the charge value, then adjusting the case capacity until the "Load Density" (percent of case capacity occupied by the powder), reads 100%, yields a "Case Capacity in Grains of H20" of 63.20 grains.

Using Alliant's Reloder 17, the case will hold 59.05 grains of powder. Setting the seating depth to 0.001", putting 59.05 as the charge value, then adjusting the case capacity until the "Load Density" (percent of case capacity occupied by the powder), reads 100%, yields a "Case Capacity in Grais of H20" of 61.05 grains.

Hmm... THEORETICALLY, the calculated case capacities should be exactly the same. The difference is 2.15 grains of water. That's a lot of error. Enough to cause a "blown" primer at least.

So I went back to "the drawing board". What assumptions was I making? I was assuming the nature of the "Load Density" calculation was LINEAR AND base on the relationship between the BD and the "Grains of H20" of the case capacity. Since I don't have access to the "guts" of QL, there was no way to test that assumption. However, "the pudding" clearly proved it wrong. The issue was that I had NOT adjusted the BD of the powder, I had adjusted the case capacity in grains of water based on the case capacity in powder, BUT QL WAS STILL USING THE "STANDARD" BD VALUE! Therefore when the next powder was used, the case capacity in water was NOT going to jibe because of the differences in BD of hte two powders.

So... The only way to get correct numbers is to measure the actual BD of each powder of interest and ADJUST that value in the powder characteristics. THEN the Load Density calculation should be correct and when compared to another powder whose BD has been measured, the proper case capacities in grains of POWDER should match.

I:
1) measured the actual case capacity in grains of water, (53.9 - Notice how far off that is from "back-calculation" using bulk density.)
2) divided case capacity in grains of powder by case capacity in grains of water yielding actual BD,
3) adjusted powder file to reflect actual BD,
4) recalculated charges for various bullets of interest.

As you can imagine, the charges for a specific MCP were considerably different - about 2.0 grains on average. THAT'S A LOT.

Here's are my points:

1) Bulk Density values MATTER.
2) QL's "standard" values are "OK" for quick and dirty calculations, but if you want the precision that QL is truly capable of, YOU HAVE TO MEASURE BULK DENSITY.

Paul

PS - I have multiple powder files. One for different temperatures, and one for MEASURED bulk densities of the powders I have in hand.

Paul

 

[DocBII]

Paul,

Just out of curiosity, you're using QuickDesign for your cartridge design?

Thanks,
DocBII

 

[gitano]

Nope. Combined with optimal barrel timing theory, just 'starting' points for reloading. Keeps me from being "hot" (most of the time ) and allows me to evaluate various parameters - like barrel length, bullet weight and bullet shape - before I commit to something.

Paul

 

[Joe R]

Paul,
I'm confused, I don't see any parameter labeled BD in QL. I've only been using QL for a year, so please forgive my ignorance. Where is it and how do you measure BD?

Joe

 

[gitano]

There is no way to input a bulk density value from the "front end" of the QL user interface. Here is a "screen shot" of how I set my QL page:

If you look at the bottom shaded box in the "Charge" window, (the window in the upper right quadrant), it is labeled "Propellant Solid Density", and in this example it has a value of 1.610 with units of grams per cubic centimeter. One must use the "Main Menu" - "Data: Add, Change, Load, Save" option followed by the "Propellant Data" option then open the "Change Data Records in Active file". (I'd show a picture, but I can't get all the windows to stay open while I "Snip" it.) Clicking on the Change Data Records in Active File opens the editor that allows modification of that specific powder's characteristics.

Hartmut Bromel put appropriate cautionary constraints on access to this part of his program. "Fiddling" with powder characteristics is ill-advised unless one knows what one is doing. However, most folks with the knowledge to understand the mechanics of calculating density, and knowing the difference between "bulk density" and "solid density" have -I believe - the ability to adjust the bulk density parameter to its true value. Doing so actually makes QL "safer" to use. I would encourage "you" to familiarize yourself with the relationship between bulk density and solid density ESPECIALLY as it pertains to gunpowder.

Due to the above mentioned constraints, there is a 'process' one has to go through to create a new powder file into which one can place one's own "adjusted" powder files. Here is a pictue of the dropdown that opens when one "double clicks" on the "File:\" at the top right of the "Charge" page.

I post that picture to illustrate two things:
1) The location of the "Powder" files, and
2) All of the different powder files I have created.
The file tree in on the right, and the list of files is on the left. Notice that the "On Hand Pwder Dens Adj.pro" file is highlighted.

One must first go to the location of the QL powder files - in my case, C:\Users\Public\Documents\QuickLOAD\data\pwders\powders and create a file. In this case, the file name I created was "On Hand Pwder Dens Adj.pro".

Back at QL, one then goes to the QuickLOAD powder file, NOT the new file you just created. The new file you just created is empty. The default QL file will have the "standard" (and inviolate) powder file created by Bromel. Open that file. (In this case, the Reloder-17 file.) THEN, open the powder file you just created.

Next, go through the process I mentioned above for editing a powder file. The editor will look like this:

The Bulk Density value is found in the box on the lower right. Put your measured value there. Use the "Save/Delete" radio button and save the file to the name you choose. (Personally, in order to minimize the chance of using the wrong data, I name my files clearly indicating that they have been modified.)

When you use the "Propellant Table Setup", QL will use your newly adjusted file. If you have only one powder in that "Powder file", there will be only one powder selected by QL in the list.

I've provided the above 'on the fly', so let me know if there is any confusion on 'steps'.

Paul

 

[BoydAllen]

Seldom have I read a post that offers up so much information. On the BD issue, you are of course aware that particularly with extruded powders the amount of powder that a case will hold, varies considerably depending on how the powder is dropped into the case, and for that reason, some standardization is likely desirable. Also, you mentioned the evaporation of "driers" as being the reason for changes over time of BD. There is another solvent that you may want to consider. A very well done test involving VV133, that was published in Precision Shooting magazine, indicated that the difference between powder that had been fully dried, and that which had been stored in a jar with water in the bottom (not touching the powder) was 2%. There was large difference in velocity obtained at these extremes using charge weights that had been measured into cases and kept there throughout the test, until firing. Powder was put in cases from a freshly opened container, and the rest of the work proceeded from there. Somewhere on my computer, I have a scan of that article. It was the first, and only of its kind that I have ever read. Keep up the good work.

 

[JarheadAZ]

Gitano;

I can independently verify that your process is exactly as described. I started doing pretty much the same thing, and reported on it several years ago in SH. It would be self-serving to call your work brilliant, but that's just exactly what it is.

Bravo.

Greg Langelius

 

[gitano]

Thank "y'all" for the kind comments.

BoydAllen - With respect to the "how the powder is dropped in the case": You are of course absolutely correct. The procedure I use to ensure the best consistency I can is to use a small plastic scoop (one of Lee's powder scoops), and tap on the case as many times as necessary until the level of the powder no longer changes when viewed by looking into the neck. Since this is the process that I use when I reload, it "works" for me with respect to getting consistent measurements.

Paul

 

[mikecr]

It seems like your resultant BD here would be w/resp to typical fill of a particular cartridge case -rather than actual for the powder.
If I did this, I would measure BD with a more suitable/standard container than a cartridge case.
Something open, less obscured, and calibrated at 100gr H20 capacity @ SG of 1.000, just to make it easier.

 

[Joe R]

BoydAllen said:
Seldom have I read a post that offers up so much information.

That's the way I feel. I don't understand half of what Paul said, but I will download it and will study it until I've digested it. It may take a few weeks or months. Another layer of the onion is revealed and more is learned.

Thank you Paul for opening this door for us.

Kindest regards,

Joe

 

[jimcasey01]

I started following this thread yesterday morning but did not respond, as I am still attempting to fully comprehend all that is covered. What I do know, it is more enjoyable and thought provoking to see than another "What is the best ( fill in the blank)?".

 

[Marksman63]

Gitano,
Could your measurements affect in some ways the weighting factor instead of tuning the BD factor?

 

[Tommie]

I see a Daily Bulletin here.

 

[BoydAllen]

I think that the underlying message here may be that manuals or computer programs are no substitute for a proper load workup because of lot to lot variations in powder, and chamber and barrel characteristics that may significantly affect pressure and velocity.

For some time, I have witnessed an increase in the number of shooters who seem to pursue their hobby on their computers and the internet, neglecting actual experimentation. I think that we all know that while various references can be quite useful, that they are not a substitute for actual testing.

Recently, a friend built a .300 WM to quite high standards, and in his pursuit of accuracy switched to a powder that seemed to have an advantage in that it filled the case more than another option that had given good accuracy. While the velocity numbers of the new powder were good, they were all over the place, and accuracy was degraded.

Often, what should be...isn't, and testing is the only way to come up with a final answer. When I am in search of a load, I load at the range, and shoot over a chronograph. It is a practice that I heartily recommend. My view of references is that they are good to have so that you can develop a list of components that are likely to be a good match for the rifle that is being worked with, but that once that has been done, all that they are good for is determining a safe starting load with which to begin testing.

 

[gitano]

mikecr - I mentioned that I had built a device to do precisely what you mention, although not with the specifics of your design. Therefore of course, I certainly don't disagree with your comments, but would add this clarification: I used the specific example to demonstrate HOW it could be accomplished for a SPECIFIC case. I have been frustrated throughout my life, both professionally and personally, when "experts" present some technical information and do not provide a REAL, PRACTICAL, example of EXACTLY "how to do it". I try not to make that same mistake.

THEORETICALLY, the bulk density measured using a specific case SHOULD be the same as that measured with a generalized measuring tool. But I agree with you that a general tool is a good choice when one is not after a cartridge-specific value, but rather a powder-specific one.

Joe R - and any others that have further questions or are confused by my comments/posts - please feel free to ask those questions either here or in a Private Mail.

Marksman63 - Your question is an excellent one and one I am afraid I am not qualified to answer in part because I consider the "Weighting Factor" to be kind of 'magic'. It SEEMS to me to be a "fudge factor", (and I don't mean that pejoratively), that Bromel inserted to account for a factor he could observe, but couldn't exactly quantify. I could be wrong in that assumption though. Anyway, I have thought about exactly what you mentioned but since I don't know from whence the "Weighting Factor" derives, I can't really tell. I am a bit skittish about fiddling with a parameter - like the Weighting Factor - when I don't know how the software obtains it.

BoydAllen - We agree, and I have had very similar experiences to the one your friend with the .300 Win Mag experienced. Like you I don't consider QL to be the "be all, end all" in reloading, and hope that is not "the message" that comes from my comments. Rather I liken its use to that of a chronograph. An indispensable tool to the reloader and widcatter, but it DOES NOT tell you "everything". For example, too many people like to say that they get max chamber pressure values from muzzle velocities (chronograph data), which is wrong "with a capital W". QL is a powerful tool. Whenever one uses a powerful tool - like a chain saw, or jackhammer, or a car - one needs to make sure they know how to use it correctly. It is even more important to "know" one's tool if one wants to "fine tune" it.

Paul

 

[Marksman63]

Gitano,
Probably you could try changing the WF and try finding a correlation with BD using your data. Just a tought for food

 

[gitano]

The fact that most of ballistics computations aren't linear, coupled with getting my fingers burned a couple of times doing ballistic "correlations", makes me once bitten twice shy. If I don't know what the non-linear coefficients of a particular function are I'm hesitant to 'fiddle'. At least for ballistic stuff.

That said, there is little doubt that one could - with sufficient input data - gain some insight into the 'magic' of the Weighting Factor. I'm chewing on your food for thought.

Paul

 

[Marksman63]

It seems the Weighting Factor tailors QL's output values to cartridges of various different shapes and sizes. The default for bottleneck cases is 0.5 while overbore cases have been assigned a value of 0.33.

What is an overbore case/cartridge? Just picked up from the AS bulletin a fair definition:
"In the simplest terms, a very big case pushing a relatively small diameter bullet is acknowledged as the classic overbore design. But it’s not just large powder capacity that creates an overbore situation — it is the relationship between powder capacity and barrel bore diameter. Looking at those two factors, we can express the ‘Overbore Index’ as a mathematical formula — the case capacity in grains of water divided by the area (in square inches) of the bore cross-section. This gives us an Index which lets us compare various cartridge designs.”

Therefore if the WF depends on case capacity, and case capacity depends on BD, it should be a correlation, even if not linear, or an empiric law which correlates WF to BD.

My 0.02

 

[aer21]

Has anyone tried calling them to see what weighting factor is?

 

[gitano]

QL also has difficulty with large-capacity, straight-walled cases, (.50-90 and other .50-??? cases, 8.15x41, and lots of the old, large cartridges), and an additional fudge factor is required to get the QL predictions for those type cartridges to match the actual data (MV adn MCP). Also small ones like .22 Hornet. I'll check my notes on that, but you can look at Bromels site for more details about it.

My efforts to get explanations of the details of the Weighting Factor have received 'foggy', "general" explanations. Essentially, "It's a fudge factor".

Paul