Recall that in the two piston driven systems we studied earlier, some of the gases generated by the burning propellant are tapped from the barrel and used to push a piston enclosed within a separate cylinder. The piston in turn acts upon the bolt carrier and bolt and causes them to cycle the rest of the action.
In a direct impingement system, there is no piston, piston rod or cylinder at all. Instead, some of the high-pressure gas is tapped out via a gas tube and piped directly onto the bolt and bolt carrier. The bolt and/or the bolt carrier are fitted tightly into the chamber of the weapon and act as a piston. In the case of the M16 family, the bolt has piston rings fitted to the back in order to provide the tight seal.
Image is taken from "US Army FM23-9, M16A1 Rifle and Rifle Marksmanship", which is in the public domain.
The bolt and bolt carrier move back due to the gas pressure. While moving backwards, the bolt extracts the fired cartridge case and ejects it via a side port. A return spring pushes the bolt and bolt carrier forward and the bolt picks up a new cartridge from the magazine on the way forward.
The following animation gives a good idea of how this mechanism works:
There are some advantages to such an action. Since there is no piston, piston rod or separate cylinder for the piston and piston rod, therefore the overall weight of the weapon is reduced. Fewer parts also means reduced costs of manufacturing. Since there are fewer and lighter moving parts on the weapon, balance does not shift as much and therefore the accuracy of the weapon is increased compared to piston driven weapons, especially in automatic mode. The gas tube is smaller and lesser mass than a cylinder containing a piston and hence it does not affect the natural resonance of the barrel as much, which also contributes to greater accuracy. Since the moving parts are in the same axis as the barrel of the rifle, this also helps performance when firing rapidly.
The disadvantages of this action are chiefly caused by the hot gases acting directly on most of the operating parts (bolt, bolt carrier, firing pin, springs, extractor etc.). The gases contain some dirty residues, which deposit on the bolt and bolt carrier and will cause reliability issues, if not cleaned frequently. This means that people need to spend more time cleaning and maintaining weapons with direct impingement actions. This also makes the weapon more sensitive to the quality of propellant used in the cartridges. The hot gases also cause the operating parts to expand. Since the parts are tightly fitted and expand at different rates, this could cause jamming and acceleration of wear and tear of the moving parts. Heat from the gases can change the physical properties (hardness, elasticity, brittleness etc.) of the metal parts and cause excess wear. The heat also evaporates the lubricant in the mechanism and this could cause the moving parts to jam up as well.
An infamous case of this is the early problems that the M16 rifles faced. When the original rifle was designed, the ammunition used for testing used a certain type of propellant (a nitrocellulose based powder). However, when the rifle was put into production and sent to the first combat units, the military found that Dupont Inc. could not produce the propellant in sufficient quantities to the specifications demanded. Hence, the ammunition was changed to use a different propellant (a mixture of nitroglycerine and nitrocellulose) which was easier to produce and met the pressure specifications. Unfortunately, this new propellant was dirtier and generated more residue than the one used in the development prototypes. Also to save manufacturing costs, the chrome-lining in the chamber, which was in the prototype design, was removed in the early production models. The early production models were also issued with no cleaning kits or maintenance instructions. These three issues caused many of the early M16s to jam unexpectedly and the rifle earned a very poor reputation during the Vietnam war. After a congressional investigation, the chrome lining was put back and cleaning kits and maintenance instructions were issued and the frequent jamming issues were resolved. While the reliability issues were solved and the M16 had better accuracy than most other competing rifles, the early bad reputation that was earned by the M16 took years to disperse.