To use your ArmaLite upper and lower, you must use ArmaLite (or compatible) barrel nut, pivot and take-down pins, bolt catch, and magazine catch. Use matching barrel and bolt (DPMS with DPMS pattern, or ArmaLite with ArmaLite). Any lower parts kit should fit with substitution of the previously mentioned parts. Start with a complete buffer tube kit, either rifle or carbine (don't piece it together or mix manufacturer's parts). If you use a free-float hand guard, makes sure it's for an ArmaLite receiver.