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'. As civilian ownership of AR-15 style rifles became sufficient to create a market for improvements, numerous manufacturers began producing one or more 'improved' modules, assemblies, or parts with features not found on factory rifles, and individuals with average mechanical aptitude can often substitute these pieces for original equipment. A distinctive two-part receiver is used by both military and sporting AR-15 style rifles. 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. Under US law, when manufactured with a barrel length less than 16 inches (410В mm) and without a shoulder stock, it is legally considered a pistol as opposed to being a short-barreled rifle, and is described as an AR-15 style pistol. As Colt continues to own and use the AR-15 trademark for its line of AR-15 variants, other manufacturers must use their own model numbers and names to market their AR-15 style rifles for commercial sale. Today, nearly every major firearm manufacturer produces its own generic AR-15 style rifle. In 2009, the term 'modern sporting rifle' was coined by the National Shooting Sports Foundation for its survey that year as a marketing term used by the firearms industry to describe modular semi-automatic rifles including AR-15s.

Some versions of the AR-15 were classified as 'assault weapons' and banned under the Public Safety and Recreational Firearms Use Protection Act in 1994 within the United States. After Colt's patents expired in 1977, an active marketplace emerged for other manufacturers to produce and sell their own semi-automatic AR-15 style rifles. In 1964, Colt began selling its own version with an improved semi-automatic design known as the Colt AR-15.

Due to financial problems and limitations in terms of manpower and production capacity, ArmaLite sold the design and the AR-15 trademark along with the ArmaLite AR-10 to Colt's Manufacturing Company in 1959. In 1956, ArmaLite designed a lightweight selective fire rifle for military use and designated it the ArmaLite model 15, or AR-15. ДЎ973 Colt AR-15 SP1 rifle with 'slab side' lower receiver (lacking raised boss around magazine release button) and original Colt 20-round box magazine Promoted as 'America's rifle' by the National Rifle Association, AR-15 style rifles' popularity is partially attributable to proposals to ban or restrict them. The rifles are controversial in part due to their use in high-profile mass shootings. In the 2010s, AR-15 style rifles became one of the 'most beloved and most vilified rifles' in the United States, according to The New York Times. ДЄfter the term modern sporting rifles was coined in 2009 by the US National Shooting Sports Foundation, a firearms trade association, it was quickly adopted by much of the industry. The Federal Assault Weapons Ban restricted the sale of the Colt AR-15 and some derivatives in the United States from 1994 to 2004, although it did not affect rifles with fewer listed features. Despite the patents expiring, Colt retained the trademark of the AR-15 and is the sole manufacturer able to label their firearms as AR-15. After most of Colt's patents for the Colt AR-15 expired in 1977, many firearm manufacturers began to produce copies of the Colt AR-15 under various names.

The then Fairchild Engine and Airplane Corporation division ArmaLite sold the patent and trademarks to Colt's Manufacturing Company in 1959. The original Armalite AR-15 is a scaled-down derivative of Eugene Stoner's AR-10 design. The part shown bottom center is the lower receiver without the receiver extension, rear takedown pin, and buttstock.ДЄn AR-15 style rifle is any lightweight semi-automatic rifle based on the Colt AR-15 design. AR-15 style rifles come in many sizes and have many options, depending on the manufacturer.

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