Chattahoochee Munitions manufactures this 338 ARC upper, a complete 10.5-inch CM15 upper chambered in 338 ARC with a 416R stainless barrel and a 1:5 twist, an adjustable gas system and a 9.5-inch M-LOK handguard. Add it to an AR15 lower to run 338 ARC, with an optional CM15 low mass bolt carrier group and ambidextrous charging handle.
338 ARC puts a heavy .338-caliber bullet below the speed of sound in an AR15-size cartridge. It needs a 6.5 Grendel Type II bolt face and a 338 ARC magazine; the lower receiver stays. See every configuration in Containment Zone 338, and read 338 ARC subsonic.
Why a 10.5-Inch 338 ARC Upper
A 10.5-inch 338 ARC upper sits near the point where 338 ARC powder use is largely complete in Chattahoochee Munitions testing, so the subsonic load gives up little while a suppressed AR15 stays compact.
The 1:5 twist adds stability for the 307 gr subsonic bullet, and the adjustable gas block tunes the upper for your suppressor.
Upper Receiver and Handguard
The CM lightweight upper is CNC-machined from 6061 billet aluminum with weight-reduction cuts and a milled channel through the Picatinny rail. The CM15 LWS 9.5-inch handguard is 6061 aluminum with M-LOK slots, a full-length Picatinny top rail and QD sling sockets. Receiver and handguard come in Cobalt Kinetics Green, FDE Metallic, Black Tungsten, Black, FDE, Sniper Grey, OD Green or Burnt Bronze Cerakote, or another color on request. B5 Systems rail covers come in Black or Coyote.
Barrel and Gas System
The 10.5-inch barrel is 416R stainless steel with a 1:5 twist, finished in E100 Blackout Cerakote, with 5/8×24 muzzle threads. The 1:5 twist spins the long 307 gr subsonic bullet 60 percent faster than the 1:8 SAAMI figure, for stability from a short barrel. The pistol-length gas system uses a .750-inch gas journal, a Superlative Arms adjustable gas block and a melonite-treated gas tube, so the upper can be tuned for subsonic or supersonic ammunition, suppressed or not. The muzzle ships with an optional muzzle brake selection or a thread protector. Read 338 ARC twist rate.
Bolt Carrier Group and Charging Handle
The optional CM15 low mass bolt carrier group uses a 9310 steel bolt with the 6.5 Grendel Type II (.449-inch) bolt face, a 17-4 stainless firing pin, an 8620 carrier and key and a hardened S7 extractor, magnetic-particle inspected. The optional CM ambidextrous charging handle is made in collaboration with Breek Arms. If you supply your own bolt, it must have the Type II bolt face, and headspace should be checked.
Magazine and Lower
338 ARC needs a magazine made for the wider ARC case; standard 5.56 magazines do not feed it. Use the 10-round 338 ARC AR15 magazine. For a complete firearm, see the 10.5-inch CM15 338 ARC pistol or SBR. Other upper lengths: 8.5-inch, 16-inch.
Suppressor Choice
With subsonic ammunition, Chattahoochee Munitions recommends an enclosed suppressor rated for .338 caliber or larger; with supersonic ammunition, a flow-through suppressor. Read the 338 ARC suppressor guide.
338 ARC Upper Specifications
- Manufacturer: Chattahoochee Munitions
- Type: Complete CM15 AR15 upper
- Chambering: 338 ARC
- Upper receiver: CM lightweight, CNC-machined 6061 billet aluminum
- Barrel: 10.5-inch 416R stainless steel, E100 Blackout Cerakote
- Twist rate: 1:5
- Muzzle: 5/8×24 threads; optional muzzle brake selection or a thread protector
- Gas system: Pistol length, .750-inch gas journal, Superlative Arms adjustable gas block, melonite-treated gas tube
- Handguard: CM15 LWS 9.5-inch M-LOK, full-length Picatinny top rail, QD sockets
- Bolt carrier group: Optional: CM15 low mass, 9310 steel bolt, 6.5 Grendel Type II (.449-inch) bolt face
- Charging handle: Optional: CM ambidextrous, made in collaboration with Breek Arms
- Colors: Cobalt Kinetics Green, FDE Metallic, Black Tungsten, Black, FDE, Sniper Grey, OD Green or Burnt Bronze Cerakote, or another color on request
- Rail covers: B5 Systems, Black or Coyote
338 ARC Upper FAQ
Who makes a 338 ARC upper?
Chattahoochee Munitions manufactures complete CM15 338 ARC uppers in 8.5, 10.5 and 16 inches, with 416R stainless barrels and adjustable gas, plus complete CM15 and CM7S 338 ARC firearms.
Will it fit my AR15 lower?
It fits a standard mil-spec pattern AR15 lower receiver. You also need a bolt with the 6.5 Grendel Type II bolt face and a 338 ARC magazine.
What bolt does it need?
A 6.5 Grendel Type II bolt face. The optional CM15 low mass bolt carrier group has one.
What twist rate does it use?
1:5 in a 10.5-inch 416R stainless barrel.
Is the gas adjustable?
Yes. A Superlative Arms adjustable gas block lets you tune for subsonic or supersonic ammunition and a suppressor.
What magazine do I need?
A magazine made for 338 ARC, such as the 10-round 338 ARC AR15 magazine. Standard 5.56 magazines do not feed it.
What colors are available?
Cobalt Kinetics Green, FDE Metallic, Black Tungsten, Black, FDE, Sniper Grey, OD Green or Burnt Bronze Cerakote, or another color on request.
What suppressor should I use?
An enclosed suppressor with subsonic ammunition and a flow-through suppressor with supersonic ammunition, rated for .338 caliber or larger.
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