Which is correct machine gun or automatic machine gun?
Machine gun or machinegun. All of them are automatic, so it is
redundant to say that. There is no such thing as a semi automatic
machine gun, or a single shot machine gun.
That answer is correct, but many people use the term fully
automatic machine gun or full auto machine gun to specify they are
not talking about a semi-auto pistol or rifle, which are commonly
referred to as autos, and sometimes that term confuses people.