Colt AR-15 - Variants

Variants

The AR-15 rifle is available in a wide range of configurations from a large number of manufacturers. These configurations range from short carbine-length models with features such as adjustable length stocks and optical sights, to heavy barrel models.

Due to the rifle's modular design, one upper receiver can quickly and easily be substituted for another. There are many aftermarket upper receivers that incorporate barrels of different weights, lengths and calibers. Some available calibers for the AR-15 platform are the .223 Remington/5.56x45mm, .45 ACP, 5.7x28mm, 6.5 mm Grendel, .338 Lapua, 6.8 mm Remington SPC, .50 Beowulf and .50 BMG. It is not recommended to chamber the 5.56x45 NATO into a rifle designated .223 Remington, due to the increased chamber pressure in the 5.56mm cartridges; the two calibers are similar, but not identical.

When installing a new complete upper receiver, particularly one designed to handle a different caliber of ammunition (i.e., other than .223 Remington or 5.56x45 mm NATO), some modification to the contents of the lower receiver may be required, depending on the particular conversion. For example, a conversion to 9 mm typically would involve the installation of a magazine well block (to accommodate a typical 9 mm magazine, such as Uzi or Colt SMG), replacing the .223 hammer with one designed for 9 mm ammunition, and depending on the original stock, replacing the buffer, action spring and stock spacer with those designed for the new 9 mm AR-15 configuration. The 9mm cartridge fires from an unlocked breech, or straight blow-back—rather than a locked breech, because the spring and bolt provide enough weight to allow this type of functioning. These guns do not utilize the direct gas impingement method of operation like the original.

Some AR-15's like the POF, LWRCI, H&K, Sturm Ruger, Sig Sauer, and Adams arms offerings replace the DGI (direct gas impingement) operating system with a short stroke/long stroke gas piston system. These guns usually have modified bolt carriers, gas keys, and gas blocks. When fired, DGI systems dump high pressure hot gas through the gas tube to the bolt carrier key and into the bolt carrier group. This can rapidly heat up the bolt carrier group and cause excessive fouling, one of the main complaints about the design. Gas piston operating systems alleviate these problems, but can be the cause of other issues such as bolt carrier tilt.

Some manufacturers offer upper and lower receivers machined from a solid billet (block) of aluminum as opposed to an aluminum forging. These include Sun Devil manufacturing, LAR Grizzly manufacturing, POF-USA, and Black Rain. This is usually done for added strength.

Upper receivers utilizing a monolithic rail system that combine a railed hand guard and upper receiver into one uninterrupted piece are made by companies like Colt's Manufacturing Company, Lewis Machine and Tool (LMT MRP), POF-USA, and VLTOR. This is done to provide a continuous uninterrupted rail section that runs along the top of the gun from the weapons charging handle to the front sight/gas block. This rail section is used for the mounting of sights, laser aiming devices, night vision devices, and lighting systems.

A side charging upper receiver has been developed by LAR Grizzly. The charging handle can be had in a left side, right side, or ambidextrous configuration. Since the charging handle is attached to the bolt carrier making it a reciprocating design, it can be utilized as a forward assist device as well.

Early models had a 1:14 rate of twist for the original 55 grain (3.6 g) bullets. This was changed to 1:12 when it was found that 1:14 was insufficient to stabilize a bullet when fired in cold weather. Most recent rifles have a 1:9 or 1:7 twist rate. There is much controversy and speculation as to how differing twist rates affect ballistics and terminal performance with varying loads, but heavier projectiles tend to perform better with faster rifling rates. Additionally, the various non .223 / 5.56 calibers have their own particular twist rate, such as 1:10, 1/11 and 1/12 for 6.8x43mm SPC, 1/10 7.62x39mm, and 1:12 for .308 Winchester.

Standard issue magazines are 20 or 30 round staggered-column magazines, traditional box magazines exist in 40 and 45 round capacities, and usable magazines have been constructed from a variety of materials including steel, aluminum, and high-impact plastics. Drum magazines with 90 and 100 round capacities exist, such as Beta C-Mags. Low-capacity magazines, usually of a 5 or 10 round capacity, are available to comply with some areas' legal restrictions, hunting, and because larger magazines can inhibit shooting from a benchrest. Surefire is now offering extended capacity magazines in 60 and 100 round capacity configurations. These magazines are a staggered column design.