This is a clever piece and well written. Unfortunately it starts with a high-concept premise and never really jumps off from there.

We get the situation quickly enough, and from that point on, it's essentially more of the same, until the final, inevitable conclusion, which offers no surprises.

You've anthropomorphized Johnny's pistol, an extreme course of action in the best of times. A possible justification might be to offer us a perspective that conventional historical accounts missed, even an alternative history. You haven't done this. So the question must be asked, "Why bother?" I don't mean to be caustic, but that's always the crucial question a writer must ask.

For example, by creating a specific character out of what would ordinarily be an inert object, you now are tasked with creating all the necessary elements that accompany any character. Goals, aspirations, conflicts, disappointments, betrayals... These are the things that make us want to know more about a character. Couldn't you find a way to provide this character with some of them? Surly he didn't expect to be used for so ignoble a deed. Perhaps he saw himself on the hip of a famous lawman, or participating in a major battle. You've given him a totally accepting response to the use to which he is ultimately put. That rings false.

Imagine, instead, that you told the tale from the POV of the man tasked with guarding the President's private box. That would offer us a familiar tale with a totally unique perspective. However he reacted, it wouldn't be to casually accept the outcome as just one of those things. Likewise, your character needs to have some kind of moral response.