## Saturday, December 26, 2015

 Image of Alliant Unique powder from National Center for Forensic Science database
As you may know from earlier posts, I reload ammunition.  One of my favorite propellents is call Alliant Unique.  It is a very old powder that is still around due to its versatility.

One source of frustration is the extremely wide range of powder loads from various sources.  This essay will attempt to demonstrate how several independent labs can work to the same specifications and reach widely differing maximum powder loads.  I will also discuss how this might impact your loading techniques.

The SAAMI spec

This is a "snip" of the actual SAAMI specs for the 9mm Luger, one of the cartridges that I reload.  The figure that is quoted most often is the Maximum Average Pressure, which is 35,000 psi in the case of the 9mm Luger.

If you look at the second column from the right you will see "Maximum Probable Lot Mean".  The footnotes tell us that the technician is to use a lot size of ten.  This is an attempt to comprehend the variation that occurs from bullet-to-bullet.

One way to calculate this is to take the average and add 1.3 standard deviations to it.  That yields the "90th percentile" number, assuming a normal distribution.

#### Distributions are rarely normal

 Image reposted for convenience.
Getting back to Unique.  Unique is a flake powder.  It is shaped much like very small potato chips.  And like potato chips, the powder column has a tendency to settle when agitated.  Being flat, they tend to pile up on ledges.  Being small, they can adhere to plastic parts due to static electricity.

I, personally, am a "tapper".  I tap the side of the measuring drum twice after it has filled up and twice after dispense while the mouth of the cartridge is still in position.  That settles the powder column in the powder measure, it knocks them off any ledges or irregularities within the powder measure, it dislodges any grains that might want to adhere due to static electricity.  By tapping the powder measure, I consistently bias the powder charge I am throwing to the maximum possible for the actual settings of the device.

Today I quantified the effect of "tapping"  I threw ten powder charges using my typical technique.  Those ten powder throws weighed 52.3 grains.  Then I threw ten powder throws using the smoothest, least agitating strokes of the powder measure that I could manage.  Those ten powder throws weighed 47.7 grains.

 Striped column is actual weight of the powder thrown.  Green column is the calculation, AVERAGE + 1.3* Standard Deviation.  Powder weight is used as a proxy for pressure.  Pressure can actually spikes dramatically as powder approaches maximum capacity.
What is counter-intuitive is that the "calculated" maximum for a "bimodal distribution" is much greater than the average...it is even higher than the larger powder charge.  A bimodal distribution is what often happens when two distinct populations are blended together.

If you were a powder company trying to sell powder on the basis of high velocities (that are consistent with SAAMI specs) you would weigh each charge to the hundredth of a grain.  Especially if your powder is a flake powder like Unique.

On the other hand, a company that is selling bullets is less likely to exercise that same, extreme level of care.  They might even game their procedure in the other direction to ensure they comprehend both tappers and non-tappers.

#### Filtering

Electronically collected data can be very noisy.  So noisy that it is not useful.

This is data that is collected from vehicles hitting stationary barriers.  The jiggly data is not raw data, it has been subjected to analog filtering...otherwise all you would see would be a big, hairy caterpillar.  The authors of this paper went even farther.  They fit the filtered data with a simple Sine fit.

A powder company that was trying to achieve the highest possible velocity for a given pressure might be tempted to more aggressively filter the piezoelectric sensor data.  That would provide the illusion that pressures were lower than the sensor actually saw.

#### So why am I a tapper?

I am a "tapper" because all of my errors will be in the direction of reduced powder charges.  Should I neglect to tap I will throw a powder charge of 4.8 grains instead of my target.  Powder charges that are 10% below the target rarely cause guns to blow up.

A non-tapper runs the risk of setting this powder measure to throw 5.2 grains, for instance, and unintended vibration will result in a powder charge of 5.7 grains.  This might not be an issue if the reloader's target powder charge is a long way from the max allowable charge (Alliant Powder lists 5.8 grains of Unique for 124 grain Gold Dot bullets) and he is shooting a stout, full size gun.  It is a script that might result in disaster if he is loading at the max and his gun not of the highest quality.

This is the kind of minutia that makes reloaders pay attention to the little details.  Tapping probably makes very little difference with more spherically shaped powders but it is a big deal with flake and flattened powders.  The little things can add up.

#### 1 comment:

1. Most people that I know using Unique are all tappers... :-)