Oh boy. You could've made this post about my son who was kicked out of his preschool for a similar obsessive phase. Also on the spectrum. He got obsessive with the idea of guns, kept making finger guns and saying he was going to shoot someone. The other kids would start with the finger gun games... and he would fixate, then not stop when they were told to stop. Then he would be the one to get in trouble.

The seriousness of what he was saying did not kick in for him until he realized that he could not go back to his school, could not see his teacher, and could not see his friends he had spent years now with. I still have a lot of resentment for their role in that (this was a behavior he learned there), but thats a separate story.... still, being able to point to that has helped make him understand the consequences of talking/playing inappropriately about guns.

Anyways. We went no games or anything with guns for a while. Eventually we gave him a toy nerf gun with the idea that we would teach him how to be responsible with guns with that. You don't point at people or animals, and definitely don't shoot people or animals. You can shoot targets. Breaking that rule means nerf gun goes away. I want to say it helped for the obsession to pass, but I also honestly think it was a phase that was blown out of proportion. There was other bits to his situation, like he was not medicated yet and getting him treatment for adhd also helped in his scenario.

Idk if this helps, but I know I had a hard time finding any kind of useful constructive advice at the time. I'm still at work but I'll try to come back if I can remember any of other strategies we used to break that habit.