They utilized a universal receiver to act as their test firearm throughout the load development process. A universal receiver allows operators to affix specially made test barrels that are fitted with Piezoelectric transducers that are specific to the cartridge the barrel is chambered for. This is for the purpose of measuring pressure. In very simplified terms, a Piezoelectric transducer works by turning the force exerted by the expanding case into a low voltage electrical charge, which is then measured by a very sensitive charge amplifier. The charge amplifier then calculates the pressure of the shot from the measured voltage and user input data from calibrations of a specific lot of brass being tested.
To make the best use of these new testing abilities, a new (for Sierra) methodology was necessary. Sierra needed to ensure that every load we listed in the manual stayed under the lessor of SAAMI or CIP Maximum Average Pressures. To accomplish this, they fired rounds to determine which load gave us the maximum pressure recommended, then they fired enough shots at that charge weight to ensure the propellant was stable at that velocity and pressure. If the propellant was not stable at that pressure, they would reduce it to a level that was. After the Max load was safely established, they worked down to a safe minimum load in two to three increments. This method determined our upper and lower limits, allowing them to interpolate in-between.
