Pressure Testing by Commercial Manufacturers

Many of our commercial reloading data sources use dedicated pressure testing rifles or pistols to get pressure data through a standard form. One of the primary motivations for this testing is to demonstrate compliance with SAAMI guidelines. Reloading data are no different – the powder or bullet manufacturer is motivated to show that their loads are safe.

Pressure detection in the commercial laboratory can be one of these three methods:

• crush cylinders,

• piezo crystals,

• strain gauge.

Calibrating piezo and strain gauge data with crush cylinder results is extraordinarily difficult other than in generic terms because the phenomena being tracked are different. Shortening of the crush cylinder takes a finite time so the change in length is influenced by the integral of the pressure history. Further, the shortening is influenced by strain hardening and strain rate effects of the copper or other standard metal during the time the high pressures are being applied.

Unlike the strain gauge method, the crystal is directly exposed to the powder gases. Couple this direct exposure to the very short response time of the crystal and we can see that this method more accurately captures the peak pressure.

The strain gauge method consists of placing strain gauges on the outside of the breech. While the gauges also have very fast response times, they depend on the response of the breech to the pressure pulse. So, while the pressure history and peak pressures obtained with this method will be very close to the piezo method, they are unlikely to be identical.

All three methods, however, produce useful results for demonstrating SAAMI compliance and promoting handloading safety.