Robert, almost all ARs uppers and lowers are milled from aluminum. That is the case here.

Robert it has a 7075 T6 aluminum forgings Lower receiver UPC:  604206910462 Description:  BSH QRC CRB 5.56 16I 30R REDDT Brand:  Bushmaster Model:  Quick Response Carbine Type:  Rifle: Semi-Auto Caliber:  5.56 NATO|223 Finish:  Matte Black Action:  Semi-Automatic Stock:  6-Position Telescoping Buttstock, Pistol Grip Sight:  No Sights Barrel Length:  16  Overall Length:  35 with Stock Extended Weight:  5.18 Capacity:  30+1 # of Mags:  1 Receiver:  Aluminum A3 Upper,*****************7075 T6 aluminum forgings Lower receiver************ Muzzle:  A-2 Birdcage Flash Hider Features:  Superlite Contour Melonite Coated 1-8 Twist Bbl; Mini Red Dot Optic, Forward Assist, Bayonet Lug

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Robert it has a 7075 T6 aluminum forgings Lower receiver
UPC: 
604206910462
Description: 
BSH QRC CRB 5.56 16I 30R REDDT
Brand: 
Bushmaster
Model: 
Quick Response Carbine
Type: 
Rifle: Semi-Auto
Caliber: 
5.56 NATO|223
Finish: 
Matte Black
Action: 
Semi-Automatic
Stock: 
6-Position Telescoping Buttstock, Pistol Grip
Sight: 
No Sights
Barrel Length: 
16
 Overall Length: 
35 with Stock Extended
Weight: 
5.18
Capacity: 
30+1
# of Mags: 
1
Receiver: 
Aluminum A3 Upper,*****************7075 T6 aluminum forgings Lower receiver************
Muzzle: 
A-2 Birdcage Flash Hider
Features: 
Superlite Contour Melonite Coated 1-8 Twist Bbl;
Mini Red Dot Optic, Forward Assist, Bayonet Lug

Hi Robert Upper receiver: Machined from 7075 T6 aluminum forgings
Lower receiver: Machined from 7075 T6 aluminum forgings

This rifle has an Aluminum A3 Upper, Aluminum Lower . (7075 T6 Aluminum).