In the dozen years since the previous writeup was written, the AR-15 has rocketed upwards in popularity, thanks no doubt in part to the expiration of the 1994 Federal Assault Weapons Ban. This highly popular semi-automatic rifle has gained a market penetration which is truly impressive. Designed by Eugene Stoner, the AR-15 is visually and functionally similar to the M-16 and M-4 used by the Armed Forces of the United States. However, it is important to keep in mind that the AR-15 sold in a gun shop or at a gun show is not a machine gun, and requires no permit to own in most states or localities. Nor is it an assault rifle, since the models available to civilians omit the selective fire capability. It does look intimidating, but that's a matter of opinion.
The design of an AR-15 is essentially linear: the heel of the stock is on the same axis as the muzzle of the barrel. The recoil generated during firing is largely transmitted back along this axis into the user's shoulder. When set upright on the stock, the rifle forms a column, with the barrel, upper receiver and buffer tube at the center. It is a lightweight design, typically made from machined or forged aluminum, with the exception of the bolt carrier group and the barrel, which are steel. This assumes the AR-15 is of a standard configuration not made for left-handed shooters. Moving the weapon back to a horizontal orientation, we place the carrying handle or railed flat-top at the uppermost point. These are part of the upper receiver.
The Upper Receiver, or upper: This houses the bolt carrier group (bolt, bolt carrier, and firing pin), as well as the charging handle. The upper receiver is open at the bottom, to mate with the lower. The right side has a trap door called a dust cover or ejection port cover. This keeps contaminants out of the upper receiver. Behind the ejection port is a shell deflector, which keeps spent casings from hitting the user. Behind this is the forward assist, a plunger which catches on grooves machined into the bolt carrier. This can be struck or pushed to ensure the bolt is locked properly. At the top is either a flat-topped section of Picatinny rail, or a carrying handle. The rear sight is usually placed onto the rail or integrated into the carrying handle. Inside the top is channel in which the charging handle is located. This engages with the bolt carrier group to pull it rearward to cycle the action manually. The actual handle portion is located at the top rear of the upper receiver.
At the front is the barrel, which extends partway into the upper receiver in what is called the barrel extension. This ensures a steel-to-steel lock up, where the bolt's locking lugs rotate to engage the barrel extension. The chamber is here. Machined into the extension are two feed ramps, to feed rounds stripped from the double-stack magazine. The barrel proceeds forward from (most commonly) sixteen to twenty-two inches. Federal law mandates a minimum length of sixteen inches, in a non-NFA-regulated rifle. The rifling will typically have a right-hand 1:9 or 1:12 twist. The barrel has a single port about midway along the top, where the gas tube is attached. The front sight is often integrated into the gas block at this port. The gas tube runs rearward to the upper receiver. Hot gas is vented directly from the barrel to the front of the bolt carrier group, where there is a small piston. This is what cycles the action while firing. Around the barrel is the two-part clamshell hand guard or barrel shroud. This prevents the user's hand from contacting the hot barrel. It is held on by the spring-loaded delta ring at the rear and a fixed retainer ring at the front, just behind the gas block. At the end of the barrel there is often but not always a flash hider or compensator.
The Bolt and Carrier Group, or BCG: The bolt is situated in the bolt carrier so that the bolt carrier will move rearward a fraction of an inch before the bolt rotates to unlock from the barrel extension, thus allowing chamber pressures to drop to safe levels. The bolt then withdraws, and extracts the spent case. The bolt has an extractor on the front face which engages the case rim to extract a spent or misfired round from the chamber. There is a spring-loaded ejector on the other side of the front face from the extractor. This throws the spent shell out of the open side of the upper reciver, once the case neck clears the chamber. The bolt is in the center of the bolt carrier, and the firing pin is in the center of the bolt. The firing pin has a rounded front point and a flat facing at the rear which is struck by the hammer. The firing pin is most often free-floating, not spring-loaded. At the top of the bolt carrier is the gas key, or piston. This fits into the rear-most portion of the gas tube.
The Lower Receiver, or Lower: Found beneath the upper receiver. The lower receiver is the part considered by the ATF to be the actual firearm, and is where the serial number must be engraved, stamped or otherwise imprinted. Oddly enough, it is the easier of the two halves to manufacture. This is the part which houses the trigger group (the hammer, trigger, sear, springs and safety). Screwed in at the rear, there is a tube called the receiver extension or buffer tube. It contains a large spring, whose purpose is to absorb recoil and return the bolt carrier group to battery. The spring is capped with a buffer, which rests on the rear of the bolt carrier group. The hammer is cocked by the rear-ward motion of the bolt carrier group. The safety is a rotating switch on the left side of the lower, and in civilian rifles has only two positions: 'safe' and 'fire'. The pistol grip is at the bottom, to the rear of the trigger. The lower receiver is also where the magazine well is located. The magazine well has a catch on the left side, which holds detachable magazines in place, with the magazine release button on the right. The button is directly linked to the catch. A loaded magazine is inserted firmly upward into the magazine well, and the bottom is struck to ensure a lock. To the left and rear of the magazine well is the bolt catch, which will hold the bolt open automatically on an empty magazine, or manually to unload the rifle. The gap between the rear of the magazine well and the front of the pistol grip is bridged with the trigger guard. At the front and rear of the lower are two pins. At the front is the pivot pin, and at the rear is the takedown pin. These lock into corresponding lugs on the upper receiver, and hold the two together. The takedown pin can be partially removed to allow the rifle to swing open around the pivot pin for cleaning and disassembly.
Ammunition: The most common chambering for an AR-15 is .223 Remington and 5.56mmx45 NATO. Other chamberings, such as .300 Blackout or 9mm Parabellum, are available. The .223/5.56 will feed in aluminum USGI 30-round magazines, or the widely-used polymer Magpul P-Mags. Versions in .308 Winchester or 7.62mm NATO are often called AR-10s.
Operation: The rifle is loaded by inserting a magazine into the magazine well. Once it is locked, the rifle is loaded by pulling the charging handle rearwards, and allowing it to return forward. At this point, the user may hit the forward assist with the heel of their hand. The rifle is now in battery. The user then aims the rifle at the desired target with either the built-in iron sights or after-market optical sights. Optics are commonly paired with back-up iron sights. The rifle is set to fire by roating the switch downwards. The rifle can be set to safe by rotating the switch upwards. With the rifle set to fire, the trigger can be pulled to fire the rifle. Unloading is accomplished by, with the rifle re-set to safe, pulling the charging handle rearward and pressing the bolt catch button. This will lock the bolt back and empty the chamber. The magazine catch is then pressed to remove the magazine. The charging handle will not return to the forward position automatically, and must be returned manually after this action.
Maintenence: With the rifle empty and on safe, the takedown pin is pressed or punched out of the lower, and the upper is pivoted upward. The charging handle is pulled rearward, pulling the BCG with it. It will can then be pulled from the rear of the upper and cleaned. The barrel should also be cleaned. Lubrication is important to proper function, therefore the BCG should be thoroughly greased. Other wear surfaces in the rifle should receive the same treatment. The rifle can be reassembled by drawing the charging handle rearward and replacing the BCG, then pivoting the rifle closed and pressing the takedown pin back in place.
I own one of these and am quite a fan. I built it from a stripped lower myself. It is quite accurate, and the design makes it easy to shoot. Mine's been customised with some Magpul furniture, which has better ergonomic characteristics, but is otherwise your bog-standard AR-15. /msg me if I missed anything.