The Walker-Wasley Critical Initiation Energy, Ec, cited by several authors as a means of comparing the sensitivity of explosives to projectile impact, is imprecise. Moreover, energy is not a necessary and sufficient condition upon which to compare sensitivities, since it might be delivered over picoseconds or years with vastly different results. Shock to detonation initiation conditions are conventionally represented by log-log relationships in Pop Plots. Historical experimental flat-fronted projectile attack data for two explosives, PBX 9404 and Tetryl were transformed using Hugoniot data into log-log form like Pop Plots. Consistent with recent research at LASL and LLNL in the US, near coincidence between such plots and Pop Plots was demonstrated. This procedure led to a method of comparing the sensitivities of these and four other explosives, two types of Composition B, TNT and Nitroguanidine/Estane (95/5), to shock initiation by projectile impact. A slight modification of this procedure can be used to generate critical projectile velocity vs. diameter curves from Hugoniot and Pop Plot data which confirm the experimental data for PBX 9404 and Tetryl. An alternative comparative sensitivity criterion to that of Walker and Wasley that is derived from Pop Plot data is proposed based on the critical power per unit mass of explosive transmitted from the initiating shock.