All players will "Shoot" when one of them does, or if a bad guy uses a gun against them.
The sounds and visual effects will all happen against every target from every player, so say my 3 players are fighting 1 big monster, when one of them takes their turn and shoots at the monster, 3 gunshots can be heard, and 3 gunshots will go off, coming from the players to the creature.
In the cases that its a cowboy shootout the same thing happens when the enemy side shoots at the players also, all 3 players shooting effects go off when they shouldn't be.
This happens weather they have matching guns or not as I have a melee fighter in the party and gunshots seem to still come from him to the target. I am not sure if anything can be done about that but I wanted to bring this to your attention.