The semi-automatic AR-15 (and the military’s fully automatic version, the M16) are, I estimate, the most ingenious rifles ever designed. The M16 is the longest-serving military rifle, having been on our frontlines since 1964. It resulted from American military planners’ desire to give soldiers a lighter, more modern rifle with easy handling qualities and the increased power and long-range accuracy of the then-new 5.56mm cartridge after the Korean War.

About the AR-15

Despite being commonly credited to Armalite engineer Eugene Stoner who developed the M16, the direct impingement gas system was first used on a French experimental rifle in 1900. In 1957, Stoner perfected the system. However, Stoner’s system is not a true direct impingement system but an expanding gas system according to his U.S. patent.

The use of the direct impingement gas system in the M16 saved Stoner an entire pound. In Stoner’s direct impingement expanding gas system, the bolt carrier essentially functions as a piston, instead of requiring a separate piston assembly and linkage. Unfortunately, the direct impingement M16/AR-15 is a “dirty shooting” rifle due to hot powder gases used directly in the operation of the action. So these require more maintenance for serious use.

How Does the AR-15/M16 Work?

One of the most concise explanations of the operating system of the M16/AR-15 comes from the Small Arms of the World (1969 Edition).

“With the weapon loaded and the selective fire (safety-fire) lever on Semi (Fire), pulling the trigger causes the (internal) hammer to strike the firing pin, which detonates the (cartridge) primer.

Once the bullet passes beyond the gas port (a part of the “tower” front sight on standard ARs, whereas flattop ARs have a gas block located in the same area) gas passes through the gas tube, and into a chamber formed by the bolt carrier and bolt. At this time, the bolt is in a locked position, acting as a stationary piston. The entering gas pressure causes the bolt carrier to move to the rear. In moving rearward, the bolt carrier rotates the bolt, unlocking and carrying it rearward.

As the bolt assembly travels to the rear, the gas is exhausted through a port in the side of the bolt carrier. The cartridge case is then extracted and ejected in the usual manner.”

How Reliable are AR-15s?

A full-length, 20-inch-barreled AR-15 rifle achieves the utmost reliability. As the barrel length decreases, the heat from the gas increases. It doesn’t have enough time to fully burn the powder. The shorter the direct impingement/expanding gas system tube, the more unburnt carbon enters the action area, threatening reliability. The short-barrel ARs foul much sooner than the rifles.

In a true piston-operated AR-15, there is no effect on the cleanliness of the rifle based on the shortness of the barrel as there is no carbon fouled gas entering the chamber. Any gas left over from operating the action is exhausted out away from it. You will need to clean and lubricate a direct impingement AR more than you do a gas piston AR, an AK-47 or a Ruger Mini-14. However, that extra maintenance requirement can help you become more familiar with your weapon. And if you understand how it works, you can better understand it’s need for higher maintenance. All in all, Stoner’s AR-15 direct impingement operating system application was a work of genius.