Gas has less affect on shooter so you can get back on the target quicker.

Your question is akward for most heavy machine guns that have been developed from the turn of the twentieth century have been recoil operated where as the recoil of the fired shell loads the next using the recoil of the gun pushing the hammer back, where as a gas operated magazine fed rifle, where as the recoil guns are belt fed the recoil cycles the belt cocking the hammer hitting the shell and cocking and rotating the belt, the gas jobs work fine they are generally lighter and not on continuous use, where the gas from the fireing shell breaks spring pressure and opens the firing bolt back cycling the rifle if that makes sense. ok one more time with the rifle loaded you press the trigger the firing shell creates gas that pushes out the bullet the springs that hold the bolt in the firing position the bolt pressue created by the springs is overcome by the gas from the firing shell thus throwing the bolt back throwing the spent shell grabbing a new bullett from the magazine and presto LOADED so fire bullet goes down the barrell spring pressure in the bolt breaks throwing the old shell casing out bolt back picks up a live round bolt pushed forward bang cycle contunues.

It depends on the weapon and caliber. Open bolt machine guns and SMGs work better with a recoil operation. Simple blowback is, well, simpler.... Gas operation has more parts and can be less effective due to fouling and poor ammunition. Most blowback weapons will fire just about anything as long as it has enough energy to operate the system. The G3 worked well but I fired it and did not like it as well as my M-16 which I didn't like much either.