It's pretty much always possible that a suspect could be going for a gun. I mean, you stop a black guy who was speeding. He reaches in his glove compartment to get registration. Or you stop a black guy because you broke his headlight. He reaches into his pocket to get his license. It's possible he is reaching for a gun. A deaf black man is in a bathroom with diarrhea. People call the police because they knock and he doesn't answer. It's possible he is in there with a gun, just waiting to finish his diarrhea, so he can run around shooting everyone. It's better to shoot them all because it's possible they have guns. Apparently.
Can we not consider some other police policy choices rather than possibly? Here are some: plausibly, viably, reasonably, probably, convincingly, definitely...