The Aero Precision M4E1 Builder Set is a matched upper and lower receiver set sold without components, allowing builders to assemble a custom AR-15 on Aero's improved M4E1 platform with their own part selections. Type: Rifle Action: Semi-Automatic Caliber: 5.56x45mm NATO Capacity: 30 Year introduced: 2016, USA Weight: 10 oz Overall length: 7.5" Status: Active Intended use: Tactical, Home Defense, Competition