A previous trigger assembly with a single embedded permanent magnet has malfunction problems such as the generation of a virtual trigger signal by an external magnetic field. In this study, we improved a trigger assembly, which can minimize the influence of external magnetic field. In order to improve the trigger assembly, two designs were considered. Through an M&S study for the optimal characteristics of Hall-effect sensor, we confirmed that the magnitude of the driving magnetic field of the Hall-effect sensor should be at least 50 gauss for the application of a trigger assembly. The second design was a magnetic bipolar simultaneous recognition system with a time-interval of 10 ms, which occurs when two embedded permanent magnets with different polarities are recognized simultaneously. As a result, the improved trigger assembly, which reflects the two design results, excluded the malfunction of small arms by the external magnetic field without magnetic shielding.