## 7/25/2018

### What is the diameter of a tree trunk?

There's a standard method for coming up with a single nominal measurement for the tapered and non-geometrical reality of a tree trunk.  First you go up 4-1/2 feet from ground level.  Then you measure the circumference of the trunk at that height.  Then you divide by pi .  What you end up with is an average diameter at an arbitrary height.  Not very meaningful in itself, but if you get that measurement and compare from year to year to year, or from tree to tree to tree, you might start to see a trend.  Careful, we might learn something.

But I didn't want to talk about trees.

Let's talk about guns!  Specifically, ammunition.  One of the meaningful metrics for the manufacture of ammunition is pressure.  Because nobody wants to KBOOM their gun.  Yet we want consistency within certain parameters from one ammo manufacturer to another, and from one lot to the next.  There came into being the Sporting Arms Ammunition Manufacturers Institute (SAAMI), where smart guys write up the "SAAMI Specs" for every recognized ammo type, from rim diameters to chamber dimensions to maximum pressures.

It takes a lot of pressure for the machine we call a gun to throw the ball we call a bullet at desired velocities, often well beyond the speed of sound.  You can't measure pressures like that with a hydraulic bourdon-tube pressure gauge.  So one question is, how do you even measure these very high pressures?

In the olden days, they came up with an indirect measurement.  They created a contraption known as a crusher gun, with a piston that communicated with a hole drilled through to barrel; the piston pushed on an arbitrarily standard artifact:  a hollow, enclosed tube of lead or copper of a known length.  The firing of the cartridge would send the piston pushing into the calibrated tube, crushing it.  Measuring that tube's length after firing gave you a relative datum known as Lead Units of Pressure (LUP) or Copper Units of Pressure (CUP).  As you can imagine, this kind of pressure testing of ammunition was expensive and slow.

During the 20th century, the development of electronic strain gauges for severe environments led to using electronic means to measure ammo pressure.  SAAMI was right there on the leading edge of that technology, and created standards for type and placement of the strain gauges for each cartridge type. These electronic instruments produce data in the form of change in electrical resistance over time, often with a considerable amount of "noise" -- but with tweaking and filtering, the Institute was satisfied with a method, enough to start calling this indirect measurement a reading of Pounds per Square Inch.

This is great!  Now for each type of loading you can look up recommended maximum pressures per SAAMI spec PSI.  What could be simpler?  What more could you want?

I dunno.  Something.  For some people, anyway.

Enter the Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives (CIP) -- Euro jerks for whom SAAMI was just not good enough.  CIP decided to take a giant step back to slow and expensive ammunition testing that uses a special (!) strain gauge that actually sits inside (!) the brass cartridge case, requiring: drilling the case, inserting that case into the chamber and lining it up with the strain gauge hole, inserting the strain gauge, securing the whole shootin' match, and then touching off the round to get the data.  (Did I mention that they are drilling! into a live! cartidge?!)

But that's not what I really wanted to talk about.

What I really wanted to ponder is the ongoing insistence that military ammo somehow has a higher maximum pressure specification than civilian ammo.  "Oh, don't shoot 5.56×45 NATO ammo in a .223 Remington gun!!!11!  5.56 is 62,366 PSI and .223 is only rated for 55,000 PSI man!"

That kind of talk makes me feel tired.

Yes, the CIP's drilled through EPVAT measurement is 430 megaPascals (62,366 PSI).  But look what you're doing there:  you're measuring the tree trunk at a different height.

Minor variations in chamber specs notwithstanding, .223 Rem = 5.56×45 if you use the same measurement methods.  Period.  The U.S. Military SCATP method just is the SAAMI method and the rounds under both names yield a pressure of 380 megaPascals Maximum, or 55,000 PSI.

Please don't confuse yourself.  You're just measuring the tree trunk closer to the ground.