This a quote from the article title above. The Evolution of the Modern Semi-Automatic
When fired with match-grade ammunition, the basic M16 is probably the most accurate standard service rifle the U. S. has ever issued. Designed by Eugene Stoner, the M16 uses a direct gas impingement operating system. This system has tremendous advantages over any of the other gas-piston systems with regard to achieving high accuracy. Basically, the gas behind the bullet is tapped well down the barrel through a hole in the barrel. It enters a tube that transports that gas back to the rifle’s receiver where it impacts an extension of the bolt carrier. When the bolt carrier is pushed to the rear, it unlocks and pulls the bolt rearward, initiating bolt reciprication.
With this system there is no operating rod, piston or gas cylinder hanging onto the rifle, rattling around, moving and imparting vibrations while the bullet travels down the barrel. It is also relatively easy to free float the M16’s barrel to take advantage of this situation, something that is nearly impossible with most other gas-operated semi-automatics. Even better, when properly set up the bullet is out of the rifle’s barrel well before any of the rifle’s mechanical parts start moving. Everything considered, Stoner’s design should theoretically be just as accurate as a bolt-action of equal quality and this is proving to be the case.