Peak Strain Meter

The Peak Strain Meter (PSM) from Fabrique Scientific, Inc. is a device which monitors the output of a strain gage attached to the barrel of firearm and displays the peak pressure in MicroStrain.  The technology used is similar to that used by the Oehler Model 43 Personal Ballistics Laboratory.  The major difference is that the Oehler unit requires the direct attachment of a computer while the gun is fired.  This would imply, for most us, that a portable computer would have to accompany us to the range.  The PSM is self contained, but only measures the peak pressure, rather than the entire firing cycle.  For most reloaders that is sufficient.  As a result of these differences, there is a significant difference in cost as well.  The current price of the PSM is $150 and fully operational strain gage assemblies is $14 each.  I purchased the PSM and 3 strain gages for less than $200.

The PSM comes with a Windows software package to convert the meter reading in Micro Strain to pounds/sq in (psi) and calculate the associated pressure curve based on measured velocity and barrel length.  The required input values to the program are:

• Barrel Outside Diameter (OD) - measured at the point of strain gage attachment
• Chamber or Barrel Inside Diameter (ID) - measured at the point of strain gage attachment
• Case Wall Thickness - measured at the point of strain gage attachment
• Barrel Length - from base of neck to the muzzle
• Bullet Diameter

Bsaed on my preliminary use of the program it becomes obvious that the accuracy of the measurements is critical to the accuracy of the calculations.

Why Measure Pressure?

For reloaders determining safe pressures in handloads is full of oportunities to make mistakes.  Ask 20 reloaders how to determine whether a load is safe and you'll get 20 differing answers with some significant overlaps. The classic things you will hear are:

• Sticky bolt lift
• Cratered primers
• Flattened primers
• Black ring around primers
• Blown primers
• Loose primer pockets
• Shiny spots on case heads
• Excessive case head expansion
• Excessive pressure ring expansion

For years these were the things I looked for to determine whether my loads were "safe" or not.  In following pressure related threads on various boards and other reading, I have come to the conclusion that there are a number of variables involved which make it difficult to be sure where you're at pressure wise.  Most of these indications apply reasonably well if you're loading relatively high intensity cartridges in modern rifles.  If, on the other hand, you're loading for older weaker actions and associated cartridges that ain't necessarily the case.

The SAAMI maximum pressures that you will find listed in various reloading resources are many times set based on the strength of old guns which may still be in use.  For example, take the 45-70.  My copy of the Lyman Reloading Handbook - 47th Edition list loads in 3 separate categories, 1873 Springfield, 1886 Winchester or 1895 Marlin, and Ruger #1/#3.  Loads for the Springfield peak at 17000 CUP, the Winchester/Marlin loads peak at just over 28000 CUP and the Ruger loads peak at 40000.  All loads use large rifle primers.  These are the same primers you woud use in modern, high intensity cartidges operating at maximums in excess of 50000 CUP safely. Do you think its likely to ever see one flatten or crater in a 45/70? I doubt it.

Different brands and lots within brands of brass and primers vary in how hard/soft they are.  This results potentially masking real high pressure loads or falsely indicating high pressures when they don't really exist if the reloader isn't careful with components.

All in all, this has never given me a real warm, fuzzy feeling.  I have always stuck with load data from documented commercial sources for this reason.

Getting Started

Assuming that you purchased the pre-soldered strain gage assemblies, or assmebled them yourself, from here on when use the term "gage", I am referring to the whole assembly not just the physical strain gage itself.  (For details on installing the gage and setting up the meter)  I took the measurements listed above as program inputs while I had the action out of the stock to attach the gage to the bottom of the chamber for esthetics.