A gas operated semi auto has very specific things that need to happen inside the gun for it to operate properly. There is the gas port size and number of them, the location at which they are placed in the barrel, the spring sizes in the gun, and a page long list of other things.

You can't go changing things in a semi auto and still have them operate properly. As a bullet travels down the barrel faster than the "normal" loads it does not allow the pressure to build properly in order for the gas pistons to cycle properly.

All of the parts in a gas operated semi auto are manufactured to operate under specific pressures and bullet speeds. If you change anything you could make the gun inoperable. Simply changing the powder charge, or bullet weight in a cartridge can make the gun not work.