System 84 Shotshell Pressure and Velocity

The System 84 is built to test shotshell chamber pressure and muzzle velocity; it has no other purpose. The Model 71 served the industry for almost thirty years, but has been discontinued.

The holy grail of shotshell velocity testing has always been the “velocity of the center of mass”. It is recognized that the size and shape of the shot column varies as it travels through the coils and downrange. That is why the industry uses “velocity coils” instead of photoelectric screens to measure shotshell velocity. Photoelectric screens normally trigger on the first disturbance through the screen. This first disturbance can be a shock wave, one stray pellet, propellant gas and particles, or just about anything else. The coil system tends to ignore the debris and to respond to the metal mass of the shot column. The coil response approximates the mass density of the shot column as it passes through each coil.

The entire signal generated as the shot passes through the coils is digitized to form a curve approximating shot density versus time. This curve is then processed to find the time corresponding to the “center of mass” at each coil. Having processed the signal from each coil, the computer then calculates the time that it took the apparent center of mass to travel from one coil to the next, and gives the velocity of the center of mass.

As a convenience to the operator, the coil response curve is displayed alongside the pressure curve for each shot. The two points that the computer determined to be the mass centers are designated with small tick marks. Any velocity number that is questioned by the gunner can immediately be verified by examining the trigger points corresponding to that velocity. Including the coil curves on the printed report extends this capability to the supervisor or anyone reviewing the report.

There are several other changes in the System 84.

  • The manual tuning of the Model 71 has been replaced by automatic tuning. The tuning curve is displayed and tuning can be manually tweaked if required by local conditions.
  • Oscilloscope snapshots are provided to verify low noise levels of the pressure and the coil signals before a shot is fired.
  • A separate oscillator module is located adjacent to the coils. This box is connected to the System 84 with RG-58 coax. Cable length between oscillator and System 84 is not critical.
  • The system uses the standard coils and spacing.
  • The System 84 has database output similar to the System 83. The system can be operated on a network, and output is in generic dBase form.
  • The operator interface is similar to the System 83 for ease of operator training.
  • The System 84 is intended for use only with Windows® 2000 Pro and Windows® XP Pro operating systems. Connection to computer is via RS-232.
  • The System 84 functions well with both lead and steel shot. The System 84 will not function with tungsten matrix or other shot composed of particles imbedded in an insulating binder.

Test Results

The sample output page shows the complete test results. A complete record of the test conditions is shown in the left column of the upper block. Complete gun and transducer information is shown in the center column. Complete load information is shown in the right column. All individual pressure and velocity results are shown for each shot. The central set of curves show the pressure/time of each shot, and the rightmost curves show the coil response from each shot. Actual trigger points are indicated by tic marks on the coil response curves. A summary block includes the statistical summary of each parameter.