Complex sentence: What was needed was a selective fire weapon firing a cartridge midway in power between the rifle and pistol/sub-machine gun, which had the accuracy of the former out to typical combat ranges, and the firepower of the latter at short ranges.
Split into simpler sentences: What was needed was a selective fire weapon firing a cartridge combiing the power of the rifle and the controllabilty of the pistol cartridges. The resulting cartridge would have the accuracy of the former for to typical combat ranges, and the firepower of the latter at short ranges.