Direct Impingement

Direct impingement describes the method by which a semi-automatic firearm—usually a rifle—cycles the bolt to eject a spent case. Direct impingement semi-automatic rifles like the AR-15 or AR-10 channel a small amount of gas from a fired round through a gas block near the end of the barrel and redirect it back to the bolt and carrier through a thin tube.  They are the most common and popular operating systems for these modern semi-automatic firearms.