Nah, they ignite in the barrel, they are just traveling faster relative to the geometry of observation; as they recede they become more visible as they no longer appear to traverse. Some heavy artillery can also be observed but you have to be close to (but not directly) behind the cannon to do it.
From the receiver point of view, active MG positions can be actually easy to find (if you don't mind keeping your head up when it is firing), dust, debris, flash and tracers can give it away.
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