The influence of microstructural variation on the initiation of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) has been evaluated. These evaluations encompass two classes of fine-grain HMX, Class 5 and Fluid Energy Milled (FEM). Initiation data of HMX using thick Polyimide flyers (203 mu m) near the power flux (pi(c)) limiting condition (2.26 GPa to 4.63 GPa) as a function of density are presented. The initiation results depict decreasing threshold velocities as density decreases for both materials. The Class 5 has a lower threshold velocity value than the FEM at comparable density values. Microstructure evaluations based solely on the total specific interface area (SIA) are inconsistent with the initiation results. Further evaluation resulted in the requirement of calculating cumulative SIA as a function of feature size to match the trends observed in threshold behavior. The results support the concept of an activated hot spot size as a function of pressure as formulated by Khasainov et al. [1]. Analysis of the microstructure indicates the distinguishing features between Class 5 and FEM are the large particle size tails of the distributions.