Hopkinson pressure bars can achieve the same mechanical pressures as falling-weight machines and have an additional advantage in that they represent a closed mechanical system that is relatively easily describable in quantitative terms. The compressive force incident on the sample can be expressed in terms of mechanical power, and it is straightforward to change the duration of loading and therefore the mechanical energy incident on the sample. We can likewise measure the amount of mechanical energy that is transmitted through the sample. In doing so we can determine that amount of mechanical energy that is dissipated/absorbed by the sample in the run up to deflagration. We find the greater the incident mechanical power, the shorter is the time to reaction. We attribute this observation to a constant energy to deflagration reaction criterion analogous to Hugh James’ description of detonation. The compact nature of a Hopkinson pressure bar also lends itself to diagnostics such as mass and optical spectroscopy for determination of reaction products and time-resolved reaction temperatures. Results from some recent studies on PETN, RDX and HMX are discussed.