Complex sentence: Most door gunners use some sort of machine gun such as the M60, M240, M2HB, or more recently, the General Electric M134 minigun which has recently made a comeback in some forms after it was phased out of military use in the late 1990s.
Split into simpler sentences: Most door gunners use some sort of machine gun such as a M60 machine gun. Replaced by M-240, M2HB, or more recently, the General Electric M134 minigun which had recently made a comeback in some forms after it was phased out of any military use in the late 1990s.