The AR 15, called by some "Lego's for Adults", is one of the most popular carbine rifles ever. The AR 15 was built on the M16 military platform and first introduced during the Vietnam police action. Part of the popularity for the AR15 rifle is its' versatility and unique ability to be customized. Simply by chaning an AR 15 upper assembly, you can completely change the rifle from a close quarters weapon to a long range tool. Therefore you can have multiple gun configurations without owning multiple guns.
Due to the popularity and flexibility of the rifle, there are also more upgrade parts available for the AR 15 than any other rifle on the market today. AR 15's are manufactured by many manufacturers including Colt & Bushmaster, and AR 15 accessories are available from manufacturers like Tapco, Trijicon, Command Arms and ProMag