The Bushmaster AR15 was first in service in the USA 1958 and is still used in present day. It was designed by Eugene Stoner and manufactured by various companies including Bushmaster.
There are a large number of differences based on the models of both guns, however the most notable is that the AR15 is a semi-automatic and the M16 is capable of semi-automatic or full automatic.
The AR 15 was designed in 1957 by Eugene Stoner. The original manufacturer of the AR 15 was ArmaLite, but they sold the rights to Colt's Manufacturing Company in 1959.
They are available for sale on websites online through proper procedures. Through proper identification and background approval the purchase of this weapon is available through online resources.
Generally speaking any gun store that has a federal firearm license (FFL) and FFL-dealers may sell an AR15. Larger stores such as Cabelas and Bass Pro shops, along with Dick's Sporting Goods, may have them in stock of be able to order.
One can buy AR 15 accessories from Rock River Arms. The company has retail stores across the globe, and they also have an online store which sells custom and pre-made AR15 accessories.
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AR-15 is short for 'Armalite, Model-15', it is a .223 caliber rifle. There are however shotguns designed specifically to be mounted on the underside of the AR-15 and guns derived from its basic design (including the M-16, M-4, and AR-15 derivatives manufactured by numerous companies).
The 'Mastery Key' shotgun from Knights Armament Company is an example of such a shotgun. When mounted to the acessory rails of an M-16 or M-4 the shotgun is primarily used as a tool for breaching doors.
An AR-15 is a rifle that uses 5.56 mm bullets. It is a semi-automatic rifle. This specific type of bullet is the most commonly used bullet amongst the NATO forces.
AimPoint CompM2, AimPoint CompM4, Trijicon ACOG, EOTech, and there has been some use of the Mepro M21.