We don't forbid, don't encourage, don't facilitate.
So if DS makes a gun out of toilet rolls and sits at the top of the stairs 'shooting' at people, that's not a problem, but nobody is going to encourage it by paying him any attention or 'playin dead' if he shoots us.
We would never buy a toy gun, but if DS and DD run round the garden shouting 'bang, bang', nobody gets told off - but DH and I don't join in, which we might well have done if it's tag they're playing.
Squeezy bottles make for great water play, but if neighbour joins in over the fence with some fancy water blaster and gives my children a turn, it isn't a problem, they just give it back to him at the end.
We don't have a games console except a shared DS with selected games. If they visit a friend and play some kind of shooting game, that's fine but we wouldn't get that game nor encourage talk about it.
DS knows a LOT about politics, the news, war (especially the second world war). He knows that guns are for killing people. He doesn't play games that are about beating people up, or stealing, or car accidents, or poisoning, or any other things that might cause death - he doesn't treat 'playing at killing people with a gun' any differently. But it doesn't have the glamour of 'being forbidden'.