Complex sentence: The gas system is fitted with a gas regulator behind the front sight base, allowing adjustment of the gas system in response to environmental conditions, and a separate gas plug can be opened completely to allow for the firing of rifle grenades and manual loading.
Split into simpler sentences: The gas system is fitted with a gas regulator behind the front sight base, allowing adjustment of the gas system in response to environmental conditions. The piston system can be bypassed completely, using the gas regulator, to allow for the firing of rifle grenades and manual loading.