While most earlier
breech-loading rifles had a single
receiver housing both the trigger and reloading mechanism, an innovative feature of the AR-15 was modular construction to simplify substitution of parts and avoid need for
arsenal facilities for most repairs of malfunctioning military rifles. A distinctive two-part receiver is used by both military and sporting AR-15–style rifles. As civilian ownership of AR-15–style rifles became sufficient to create a market for improvements, numerous manufacturers began producing aftermarket parts—including parts with features not found on basic AR-15 rifles, and individuals with basic mechanical aptitude can often substitute these pieces for original equipment without needing a gunsmith. Due to the vast assortment of aftermarket parts and accessories available, AR-15–style rifles have also been referred to as "the Swiss Army knife of rifles", "Barbie Dolls for Guys", or "LEGOs for adults". These more or less
interchangeable modules are a defining characteristic of AR-15–style rifles. The lower receiver includes the
trigger guard in front of the detachable
pistol grip, and behind the magazine well. Lower receivers may be bought "stripped"—a single solid part and legally a firearm in the United States, albeit nonfunctional, with no fire control group or lower parts kit installed. End users may install their own choice of fire control group and lower parts kit. The lower receiver holds the
trigger assembly including the
hammer, and is the attachment point for the
buttstock. The lower receiver is attached to the upper receiver by two removable pins. Disassembly for cleaning or repair of malfunctions requires disengaging these pins from the upper receiver. Releasing the rear take-down pin allows the receiver to be opened by rotation around the forward pivot pin as a
hinge. A few manufacturers have made full upper receivers, or even "bufferless" bolt carrier systems where the buffer system is wholly contained in the upper receiver, and therefore does not use the buffer tube, which allows for firing while the stock is in the folded position, or removal of the stock altogether.
Gas systems The standard design includes a gas block and tube to vent burnt powder gas back into the bolt carrier assembly where it expands in a variable volume chamber forcing the bolt open to eject the spent cartridge case. The buffer spring in the buttstock then pushes the bolt closed after picking up a new cartridge from the magazine. This
Stoner bolt and carrier piston system has the disadvantage of venting un-burned
smokeless powder residue into the receiver where it may ultimately accumulate in quantities causing malfunctions. Some AR-15–style rifles use an alternative
short-stroke gas piston design borrowed from the
ArmaLite AR-18, where a metal rod pushes against the bolt carrier, driven by a piston located just behind the barrel gas port. This piston design keeps the rifle cleaner by not exhausting in to the receiver. Other AR-15–style rifles feature redesigned gas systems so the rifle is "over-the-beach capable", allowing it fire safely as quickly as possible after being submerged in water. The original design features a free-floating
firing pin. To theoretically reduce the risk of
slam-firing, the HK416 and its civilian variant MR556 feature a proprietary firing pin safety in the bolt. Such firing pin safeties may obstruct the upper from working with standard AR-15-type full height hammers located in the fire control group of the lower.
Left-handed users Most rifles eject spent cartridges from the right side of the receiver away from right-handed shooters who place the butt against the right shoulder while sighting with the right eye and using a finger of the right hand to pull the trigger. Right-side ejection is a disadvantage for the third of the population whose left eye is
dominant, and for the tenth of the population who are
left handed, because holding these rifles against their left shoulder for maximum accuracy may cause the rifle to eject hot spent cases toward the chest, neck, or face of a
left handed shooter. When the M16A2 was adopted by the Army in 1986, it incorporated a built-in brass deflector to keep ejected cartridges from hitting the user. Most civilian variants also copy that feature. The modular design of AR-15–style rifles has encouraged several manufacturers to offer specialized parts including leftward ejecting upper receivers and left-handed bolts/bolt carriers for converting right-handed AR-15–style rifles for left-handed use. Ambidextrous lower receivers, magazine releases, and safety selectors have also been produced, allowing release of the magazine from the left side, closing of the bolt from the right side, and operation of the safety from the right side, respectively.
Calibers The AR-15 is nominally chambered in
.223 Remington or
5.56×45mm NATO, with the
.223 Wylde chamber allowing for the safe chambering of both, but many variants have been produced in different calibers such as
.22 LR (sometimes referred to as an AR-22 ),
7.62×39mm,
9×19mm Parabellum,
6.5mm Grendel, and shotgun calibers. Some of these firearms chambered in smaller calibers such as 9mm or .22 utilize
simple blowback or
delayed blowback operating principles instead of the default direct impingement/internal-piston based operating system, as insufficient gas pressure or volume is produced by the round to cycle the action, or the simpler blowback system is sufficient and may allow removal of the rear buffer tube and spring.
Compliance with state or local restrictions Some AR-15–style rifles limit use of detachable magazines to comply with state regulations. Nearly all versions of the civilian AR-15 have a pistol grip like the military versions, but as the pistol grip is generally removable, grips and stocks that comply with various restrictions are available. While AR-15–style rifles are banned in
New York City, the FightLite SCR has been explicitly allowed, even though it accepts standard AR-15 upper receivers. The SCR lower receiver differs from the standard AR-15 lower receiver in that it uses a
Monte Carlo stock instead of a pistol grip, which may allow it to be legally possessed in jurisdictions with assault weapon restrictions in place. It also uses a proprietary bolt carrier due to the angled buffer tube, and a proprietary fire control group that moves the trigger rearward. A few manufactures offer
bolt action or
pump action AR-15–style rifles incapable of semi-automatic fire. These are most commonly marketed in jurisdictions where ownership of semi-automatic centerfire rifles is heavily restricted, such as
in the United Kingdom and Australia. == Comparison to military versions ==