Determining Case Fill Percentage


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lencac
July 21, 2013, 11:52 AM
Hi guys.
Ok, when I see in certain loading manuals they sometimes list case fill percentages.
My question is this: When that percentage is calculated for bottleneck rifle cartridges is that percentage figured based on the 100% volume as measured from the very top of the case, the top of the shoulder or from the bottom of the shoulder? :scrutiny:
Thanks:)

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Jesse Heywood
July 21, 2013, 01:39 PM
How To Measure Case Capacity.
Measuring case capacity is actually quite simple. Once you have established your bullet seating depth in the bullet section of this guide.
1. Weigh one case with bullet seated to proper depth without powder or primer.
2. Now fill the case with water through the primer hole using a hyperdermic needle and weigh again.
3. Now subtract dry weight from the water weight and this will give you your case capacity.
Example:
470 grains = water weight with bullet.
- 420 grains = empty weight with bullet.
50 grain = case capacity.

Once you have your case capacity using your reloading manual select powders that fall into your load density range.
In the above example of a 50 grain capacity your range would be.
a. 50 grain capacity x 80% load density = 40 grains of powder.
b. 50 grain capacity x 90% load density = 45 grains of powder.

lencac
July 21, 2013, 01:54 PM
Jesse .............. excellent :cool:
That makes perfect sense. :scrutiny:
So the load density is not based on a static measurement of the case but rather a variable factor based on the particulars of a couple different variables. :)
You are da man, man ;)

beatledog7
July 21, 2013, 07:06 PM
But that only works if the powder's bulk density is 1.000, the same as water's. It's not. Close for some powders, but not 1.000.

I suggest you read this:

http://stevespages.com/powderdensity.html

StretchNM
July 21, 2013, 07:42 PM
Yes, but it's close enough when the idea for accuracy is generally to fill as much of the case as possible. Lee's manual is good at giving "useful case capacity" under the cartridge drawing of each caliber. It gives the measurement in CC's, but then CC's are one of the measurements listed in the tables. So, a little light math and all is pretty good.

If, using the Lee manual, you did your math and used a powder that you thought would give 90% fill, but in the end it was so full and compacted that the bullet wouldn;t stay seated, well....you would learn from that.

beatledog7
July 21, 2013, 08:21 PM
Agreed in principle, StretchNM. I'd use available density data if I had no powder to test and was going shopping.

But if I have on hand three or four plausible powders for a given round, it would be faster and effectively as accurate to simply pick the charge for each powder that will produce the desired velocity, charge one case accordingly for each powder, and visually inspect them. The one that's fullest has the lowest density powder; the emptiest one has the densest powder.

If you know what OAL you'll be using, you can pretty much tell where the base of the bullet will be WRT the top of the powder charge; therefore, you can get a sense of how much of the case is going to remain empty. This is of course not scientific, but it's pretty useful if one only needs to be able to tell the difference between 75% and 90%.

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