The AR-9 is a modified version of the AR platform, developed by Colt in the 1960s in an effort to adapt the AR-15 to a smaller, personal defense weapon. It never took off, as Colt did not want anything competing with it's own AR-15 line of rifles; however, it is still a magnificently functioning weapon based on a tried and true platform.
The AR-9 is a tightly controlling subgun. While it has the low damage of a 9mm, it has a high rate of fire, as well as magnificent stability and very good accuracy.
The AR-9's mod pool does not quite meet that of the AR-15's. However, with support from other mods, including other AR Pack modules, you will never run out of customization options.