Mikeal, The Frame Material is Aluminum Alloy. smith-wesson.com/firearms/mp-15-sport-ii

7075 Aluminum alloy. It is very strong and rigid but you cannot easily modify or repair it (read weld it) like 6061 alloy. It tends to be a bit more brittle than 6061 but has better wear and chemical resistance.

7075 aluminum I have one and it has been a great starter AR 15 (all I need) Model M&P 15 Sport 2 OR 5.56mm 16 Inch Threaded Barrel Matte Black Finish 30 Round
Smith & Wesson M&P 15 Sport 2 OR
Detachable magazine. Threaded sixteen inch Armornite coated barrel. 7075 Aluminum receiver. 4140 Steel barrel material. Polymer grip. 6-position adjustable stock. Gas block with integral pictinny-style rail. QD sling swivel attachment point. Chromed firing pin. Forged, integral trigger guard. Foward assist and dust cover. 1 in 9 inch twist. Matte black

Mikeal, both the upper and lower are made of 7075 T6 Aluminum. Specifications and Features: - Smith & Wesson Military & Police 15 Sport II Optics Ready 10159 - AR-15 Semi Automatic Rifle - Direct Gas Impingement = 5.56mm NATO caliber (accepts .223 Remington) - 16" B4140 Steel Barrel - 1:9" Twist - M4 feed ramps - 7075 T6 Aluminum upper and lower receivers - Mil spec hard coat anodized black - Railed Gas Block - Magpul MBUS folding rear sight - 6 Position telescopic stock - Magpul MOE Carbine Length M-LOK Hand Guard - Chromed Gas key and bolt carrier - 30 round magazine - Overall length 36.8" Extended / 32" Collapsed - Overall weight 6.45 lbs - Made in the USA - Matte Black.