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18″ MARK12 Complete Upper Barrel Receiver Assembly

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The Luth-AR 18″ Mark12 Complete Upper Barrel Assembly

Starts with our 18″, 5.56mm Mark12 barrel made from 4140 Chrome Molly with a 1-9″ twist mated to our A3 Flattop receiver which includes our bolt carrier, charging handle, forward assist and ejection port cover. The upper is then topped off with our lo-profile gas system and our ergonomic  15″ Palm Handguard® with KeyMod slots and a Picatinny top-rail.

Currently our Complete Upper Barrel Assemblies are out-of-stock.



The Luth-AR 18″ Mark12 Complete Upper Barrel Assembly


  • HG-V-15K – Palm Handguard® – Vented 15″ KeyMod with Barrel Nut
  • BAA3-M12 – 18″ Mark12 Barrel
  • FTT-EA1 – A3 Upper Receiver Assembled
  • BC-A-223 – Fully Assembled Bolt Carrier – .223
  • BL-04 – Standard Gas Tube
  • GB-LP750 – Gas Block .750
  • UR-19 — Charging Handle
  • BL-06-A2 — A2 Flash Hider

Upper Specs:

  • 7075-T6 Forging
  • Hard coat anodize Black

Bolt Specs:

  • Bolt: Carpenter 158 steel
  • Extractor: 4140 Alloy steel
  • Army Spec Heat Treatment
  • Manganese Phosphate

Bolt Carrier Specs:

  • Carrier: 8620 steel
  • Cam pin: 4340 Alloy steel
  • Carrier key: 4130 Alloy steel
  • Army Spec Heat Treatment
  • Manganese Phosphate

Barrel Specs:

  • 18″ Mark12 Barrel
  • 5.56 mm
  • 4140 Chrome Moly
  • Threaded Barrel — Thread Pitch 1/2 x 28
  • 1-9 RH Barrel Twist
  • 6 Lands and Groves
  • M4 Feed Ramp
  • Button Broached


SKU: BAA3-M12 — KeyMod




.556 mm

Slot Style

KeyMod, MLOK™, Smooth


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For best results we recommend following a break-in procedure. We recommend that you clean the chamber and bore after every round for the first 25 rounds. It should take approximately 200 rounds for optimum barrel accuracy. Please keep in mind that accuracy is dependent upon many factors such as bullet weight, powder load, etc. If you have any question regarding our procedure, please ask or give us a call.
Per ammunition guidelines it is not recommended to shoot 5.56mm in a .223 REM  barrel, which we support for best and safest use of our product.