Complex sentence: During the Vietnam War, the US Navy SEALs used the Carl Gustav m/45 SMG extensively, as one of its qualities is that it can fire almost immediately out of the water (over the beach).
Split into simpler sentences: During the Vietnam War, the US Navy SEALs used the Carl Gustav m/45 SMG extensively. One of the guns qualities, which appealed to the Navy was that the m/45 can fire almost immediately out of the water (over the beach).