Complex sentence: Although this suggestion was fruitless, the US Army later engaged in many studies of the 6mm SAW cartridge for the same purpose of replacing rifle and machine gun cartridges with one round.
Split into simpler sentences: Although this suggestion was fruitless, the US Army later engaged in many studies of a 6mm SAW cartridge. They, once again, sought to replace autoloading rifle and machine gun cartridges with one round.