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. Some manufacturers are ArmaLite, Bushmaster, Doublestar and DPMS.
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Due to the AR-15 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. This includes drop-in .22 LR conversion systems which come fully-assembled and ready-to-install in your AR-15 to help you practice your critical shooting skills with more economical rimfire ammunition.
Other available calibers for the AR-15 platform are the .223 Remington/5.56x45mm, 6.5 mm Grendel and 6.8 mm Remington SPC. Care must be taken not to chamber the 5.56×45 NATO into a rifle designated 223 Remington, as it may cause an unsafe high-pressure hazard given that 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 5.56×45 mm NATO), some modification to the contents of the lower receiver may also 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.
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 for 6.8x43mm SPC and 7.62x39mm, and 1:12 for .308 Winchester.
Standard issue magazines are 20 or 30 round staggered-column magazines, traditional box magazines also 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 also 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.
Source: Wikipedia contributors, “AR-15,” Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=AR-15&oldid=342294830 (accessed February 8, 2010).