I think the M60 and RPD are only more visible because they have a near constant glow from the distance when firing.
A CAR-15 will quickly become noticed if you're alone and prone on the side of a hill. Don't really worry though, the flash in the distance is the same from both guns. Use the CAR-15 if you like it because you'll probably actually make the kill unlike the guy spraying with RPD.
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