Most of live military training systems are based on the MILES gear. In order to simulate the real engagements, it simulates a gunfire using a laser beam, and death by being shot is judged by sensors, which are attached on the trainer's body. Therefore, one of the important design consideration is to choose proper specifications of MILES gears to embrace the properties of real firearm. To decide the specification of MILES gear, the designer should decide several things. The problem is that conducting this with real experiments needs a lot of time and cost because of too many combinations of specifications. This paper suggests an optical engineering simulator to compute an efficient design of MILES gear. The simulator is based on the domain knowledge of the laser beam and the sensors to acquire high fidelity results, so that the designer of MILES gear may find the proper specifications easier.