Complex sentence: The principal medium machine gun was the Breda M37 but it too had its shortcomings, the major one being its 20-round cartridges, which gave it a reduced rate of fire.
Split into simpler sentences: The principal medium machine gun was the Breda M37. It too had its shortcomings, the major one being that it used 20-round cartridges, which gave it a reduced rate of fire.