Here is a link to complete build kits, everything needed to complete your lower receiver in a variety of calibers and barrel lengths. There is 5 pages, please make sure you select the right caliber and barrel length. They start at 316.10 USD
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TacFire's AR-5.56 Unassembled rifle build kit includes a black parkerized barrel with a 1:8 twist rate, 1/2x28 A2 flash hider; a Slim M-Lok free-float handguard; 7075-T6 billet aluminum stripped upper; 5.56 bolt carrier group; Mil-Spec charging handle, dust cover, forward assist, carbine length gas tube, low profile gas block; complete mi-spec M4 stock and lower parts kits all made in the USA. This model has a 16" barrel and 15" handguard.
Please click picture for link to site
TacFire's AR-5.56 Unassembled rifle build kit includes a black parkerized barrel with a 1:8 twist rate, 1/2x28 A2 flash hider; a Slim M-Lok free-float handguard; 7075-T6 billet aluminum stripped upper; 5.56 bolt carrier group; Mil-Spec charging handle, dust cover, forward assist, carbine length gas tube, low profile gas block; complete mi-spec M4 stock and lower parts kits all made in the USA. This model has a 16" barrel and 15" handguard.