Load density and case capacity.

RoscoeT

RoscoeT

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Being new to reloading I am reading everything I can.

Quoted below are parts of an article that I found. The part I had trouble with was calculating case capacity. It seems to assume that water weighs the same or has the same density as all types of powder.

Is this guy out to lunch or is this a standard way of measuring case capacity and load density?



http://www.gunnersden.com/index.htm.rifle-reloading-powder.html
Load Density
<snip>
Load density is the ratio between case capacity and actual powder charge. Most factory ammunition is loaded with a density of 80% - 90% of the cartridge case capacity.
<snip>
How to figure load density.
Powder charge weight divided by case capacity equals load density.
Example: Powder charge 40 grains (divided by) Case capacity 50 grains = .8 or 80% load density.

Case Capacity
Case capacity is the amount of volume inside a cartridge case that is available for the powder to fill.

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.

a. 50 grain capacity x 80% load density = 40 grains of powder.
b. 50 grain capacity x 90% load density = 45 grains of powder.
Click to expand...
 
He seems to have it right. You'll see powders with densities listed as "0.910" for example. That means that it weighs 91% as much as an equivalent volume of water, so you could only fit 91% as much into the case. If you desire 90% load density then for the case with 50 grs of available space you would load 82% (90% X 0.910) of the 50 grs = 41 grs of powder.
 
Andy said:
He seems to have it right. You'll see powders with densities listed as "0.910" for example. That means that it weighs 91% as much as an equivalent volume of water, so you could only fit 91% as much into the case. If you desire 90% load density then for the case with 50 grs of available space you would load 82% (90% X 0.910) of the 50 grs = 41 grs of powder.
Click to expand...

Ok, now that makes sense. If you have the density of the powder vs. water to work with then you can figure it out.


Thanks for clearing that up.

Roscoe
 
keep reading. keep asking.

The more practical part of this is that you don't want a laod that only fills, say, half the rifle case with powder. Ignition and prssure could be eratic. One likes to fill a case to close to the bottom of the shoulder, or higher.

I like loads that are lightly compressed. Ignition would probably be more uniform.

When we load fast pistol power in roomy pistol cases, the case can by 75% empty, and pressure can vary depebding on where the powder is at time of shooting. Not good.
 
I load what shoots most accurately, and that tends to find me in the 95% load density range which is often a max pressure load.
 
He has sort of lost me in that figuring, also. I don't get where has equated water weight with powder weight, or density.
However, this is a typical case of a writer making a very simple matter complicated.
As Ganderite has stated, use a powder that gets up to full pressure, with the bullets used, when the case is full, or very nearly full. Or, a bit compressed, as G says.
Put the charge of powder in the case. Look in the case. If it is about full, great. If there is considerable space between the powder and where the base of the bullet will be, use a slower burning powder. Who cares if it is 94% full or 98%. It just should be nearly full.
The cartridge is most efficient with a powder that will fill the case with a top load.
 
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