In firearm terminology, “gas operated” refers to a method of readying a weapon to fire immediately after it discharges a round. To understand this process, you must know how a bullet fires. While there are thousands of firearm designs, most utilize ammunition that houses a projectile, a propellant consisting of a combustible material and a shock-sensitive explosive primer inside a cartridge. In most modern designs, pulling the trigger of the loaded firearm releases a spring-loaded bolt. The bolt contains a firing pin that collides with back of the bullet, setting off the primer. This ignites the propellant and forces the bullet out of the barrel.
In both fully automatic and semi-automatic rifles, the bolt is spring-loaded so that its natural tendency is to move forward. Releasing the trigger allows it to do so and push a round into the chamber and send the firing pin into the primer. Semi-automatic weapons prevent the bolt from cycling “automatically.” That is, the shooter must release the trigger and pull it again. Fully automatic weapons allow the bolt to cycle as long as the trigger is depressed.
Gas-operated machine guns include a tube above the barrel. At one end of the tube is a “tap-off” hole that allows air to pass from the barrel to the tube. The other end leads to the bolt. When the weapon fires, the tap-off hole diverts into the tube some of the forward gas pressure used to fire the bullet. This pressure travels down the tube and into the bolt, forcing it backward. Spring tension sends the bolt forward again while a spring in the magazine forces a new bullet into an area near the barrel called the “breech.” The bolt pushes the round from the breech into the barrel and sends the firing pin into the primer, starting process over again. This is known as the Direct Impingement (DI) method.
The problem with the DI system is that the gas is hot and filled with carbon and other impurities. The heat can put strain on the moving parts that make up the bolt assembly. Additionally, the impurities can cause these parts to clog. As these are among the most critical components, the weapon becomes subject to more wear and tear and requires regular cleaning and maintenance.
In an effort to increase reliability with minimal maintenance, some gas-operated firearms employ a piston. These function similarly to DI systems except the gas pushes on a piston housed where the tube would otherwise be. The tap-off hole diverts the gas pressure into the piston, which, in turn, pushes the bolt. As such, the gas itself does not come into contact with the critical parts in the bolt assembly.
It is not entirely accurate to say that gas piston firearms run cooler and cleaner than their DI counterparts. The area that houses the bolt assembly stays cooler and cleaner but only because the heat and fouling materials have been moved partway down the barrel where the tap-off hole diverts the gas into the piston. This area can get just as hot and dirty as the bolt otherwise would and as such requires cleaning and maintenance.