Firearm Gun Uppers and Conversion Kits for ARs and other rifles. Building your own gun? Looking to change the look of your current AR-15? Click-Click-Boom has hundreds of upper receivers for your rifle as well as tons of conversion kits for your guns. All of the firearm conversion kits found here are manufactured to be compatible with a wide range of guns and come complete with everything you need to change from a 5.56 to a .300 or any other popular conversion.