To verify a hypothetical failure cause of a gun-launched missile's solid rocket motor (SRM) under conditions of high environmental temperatures and overloaded forces of up to 6000 similar to 12000 g, experiments and simulations were conducted and a new criterion variable was proposed. Uniaxial compression tests of the composite modified double base (CMDB) propellant used in the gun-launched missile were carried out at low and intermediate strain rates at 50 degrees C to characterize the CMDB propellant's dynamic properties. The Schapery nonlinear viscoelastic constitutive model was employed and applied in finite element analyses. Contact and an oscillating frictional direction between the grain and the case were observed when the overload curve oscillated with time. Three key variables of the process were researched: the contact pressure, the relative velocity, and the total contact time. Based on these, a new criterion variable q is proposed for judgements regarding the SRM's thermal safety. By comparing the results with those from friction sensitivity tests, it was discovered that the grain had a possibility of self-ignition that cannot be ignored. Meanwhile, the gap width between the case and the grain was determined to be a key influencing factor.