Signs of Pressure

This topic has been a burr under my saddle for a while. I have regular conversations with handloaders and other shooters who nonchalantly discuss “signs of pressure” and understand them to be “sticky extraction” “ejector marks” “flattened primers” and the only “sign” that anyone seems to take seriously is “primer pocket expansion” to the point that cases cannot be reused…because cases are expensive…  Since when did short case life matter more than firearm longevity or personal safety???  

I was super lucky.  I got to learn about internal ballistics in person from industry legends and hands on with state of the art P&V equipment.  The first industry legend I got to study under is a man named Ken Oehler. He built the footings for my understanding of internal ballistics – before that, he built a company called Oehler Research.  I picked up a lot of the rest of my knowledge from working in a ballistics lab on a reloading manual project under the leadership and guidance of a guy named Dave Emary.   

When I first met Ken Oehler at his range near Fredricksburg TX, he asked me a question that has stuck with me since.  I was there to learn how to use strain gauges to accurately measure chamber pressure for the express purpose of creating the most precise reloading data possible.  The year was 1999.  We were discussing using his Oehler M85 strain gauge system.  Ken asked, “What is the first sign of pressure?”  My response was, “very slightly sticky bolt lift.”

Ken responded dryly, “No. The first sign of pressure is the bullet exiting the muzzle.” 

That set the tone for the rest of my time learning from Ken.  The lessons he taught me stick with me today.  Firearm chamber pressure is not a linear black and white thing and it darn sure isn’t something that can be measured by the way a bolt handle lifts.  Two different cartridges loaded to the same pressure can make cases look and act completely differently due mainly to the manner in which the pressure is applied.  

In the days prior to Piezo Transducers and strain gauges, lead and copper cylinders called “crushers” were used to measure pressure in Lead Units of Pressure (LUP) and Copper Units of Pressure (CUP).  While accurate and relative, CUP and LUP failed to capture more dynamic events that are very readily visible with Piezo-electronic pressure measurement or strain gauges.  Time under the curve – or the duration which the pressure is present – has an effect on the way the copper and lead crushers presented data.  Quicker burning propellants tended to make pressures look higher than slower burning propellants.  CUP and LUP use a phenomena called “plastic deformation” to define the pressure created by different cartridge/bullet/propellant combinations.  CUP and LUP ARE NOT the same as PSI, nor is there a correlation. 

When handloaders talk about seeing “signs of pressure” when a big ejector mark shows up, what they’re really talking about is the material threshold where the brass cartridge case begins to fail due to a thing called “Plastic Deformation”. “Plastic Deformation” is when a material changes shape or form beyond the point of “Elastic Deformation” and is permanently changed.  Under “Elastic Deformation” the object will return to its original shape. 

“Plastic Deformation:  is a permanent deformation or change in shape of a solid body without fracture under the action of a sustained force.”

“Elastic Deformation:  is deformation that disappears upon removal of the external forces causing the alteration and the stress associated with it.”

Much is expected from brass cartridge cases.  Cartridge cases are expected to hold the bullet, the primer and the propellant securely and precisely.  When fired, they are expected to expand to fill the chamber and seal 50,000 – 60,000psi of hot, angry, gas moving at up to 7,000 fps from coming back into the shooter’s face.  The case then retracts slightly to eject from the chamber, then repeat that task over, and over again as many times as it’s reloaded.  Over time, plastic deformation will accrue and render cases in need of a trim…eventually, they will wear out and become unusable.  The pressure to which the cases are loaded will determine the amount of times which they may be reloaded.  Cases loaded to reasonable pressures, within the threshold of the material from which they are manufactured will last a long, long time.  Cases loaded beyond the threshold for plastic deformation may, or may not, be able to be reloaded.  In extreme cases loading to the point of catastrophic material failure puts both shooter and equipment in danger.  Where is this precise line?  It’s honestly hard to say exactly where the line between relative safety and catastrophic failure lies.  BUT it is safe to assume that this line lies a safe margin inside loading data that can be found in reputable, published sources that has been tested and vetted by experts.

So….why do shooters still use “guess & by golly” methods to determine chamber pressure?  “Why” is the correct question to ask in this situation. 

If one is shooting a 308 Winchester and only getting 1 or 2 reloading cycles from the cartridge cases before the primer pockets become loose….one might be wise to accept the fact that they are shooting a 308 Winchester and load it accordingly ….OR…. consider trading it in for a 300 Win Mag.

“The first sign of pressure is the bullet exiting the muzzle.”