Modern Firearms that are Gas Operated often have Gas Plugs, Pistons or regulators to push the Bolt back, reload and fire again, all in a split second.
A Good Example is the M240 / L7 / GPMG / C6. The Gas setting is set on the Firing Range using a set procedure.
The ability to regulate the Gas pressure allows you to fine tune the Rate of fire, but it also allows you to reduce the strain or damage on the weapon from Sustained fire. Firing your weapon without "Balancing it" leads to excess wear on it and reliability issues.
Equally the SA80/ L85A2 has a Gas Plug setting of "E" For Excessive if your weapon has been used for alongtime / Significant Engagement and this increases the Exhaust size of the Gas Plug and stops the Weapon becoming blocked by a build up of Carbon and burnt propellant.
Much like we can adjust Zero Distance, it would be good on sustained fire weapons if we could adjust the GAS PLUG Settings, as this would alter the Rate of Fire and the controlability of sustained fire or Automatic weapons.