Other players can't see the Gun at my back?

If the gun is created on your back or wherever on the client only you can see it no one else.

If that is the case you should create a RemoteEvent and have it create the gun on the server.

Just if you don't know here's how to use a RemoteEvent:

-- Create a script (not a localscript) inside the RemoteEvent
script.Parent.OnServerEvent:Connect(function(plr) -- When you fire a RemoteEvent from the client the first arguement will be the player who fired it
end)

I hope this helps!