Microphone Settings to Record Loud Shots

I am using a shot timer app (Free Shot Timer) that is designed to signal a pistol shooter when to draw their gun and then record the time each shot is fired. The problem is the built-in microphones on my iPhone XR don't respond to the gunshot - presumably due to noise cancellation to protect hearing. I have tried to adjust the microphone settings without luck. If I plug in a pair of lightning earplugs, the iPhone will detect the shots. Does anyone know whether the iPhone XR can be adjusted to enable the built-in microphones to detect a gunshot and, if so, exactly what settings to change? Thanks!