Complex sentence: The Chinese claim that they have tested their new cartridge extensively against both the 5.56x45mm SS109 and the Russian 5.45x39mm 5N7 and that the 5.8x42mm outperforms both cartridges with penetration superior to the SS109, a flatter trajectory, and a higher retained velocity and energy downrange.
Split into simpler sentences: The Chinese claim that they have tested their new cartridge extensively against both the 5.56x45mm SS109 and the Russian 5.45x39mm 5N7. The 5.8x42mm outperforms both cartridges with penetration superior to the SS109, a flatter trajectory, and a higher retained velocity and energy downrange.