CM15 338 ARC AR Upper – 8.5″ Compact Upper
The CM15 338 ARC AR Upper is Chattahoochee Munitions’ shortest current 338 ARC upper assembly for the AR-15 platform. Built around a lightweight CM billet receiver, 8.5-inch 1:5 twist 416R stainless-steel barrel, 7.25-inch M-LOK handguard, and adjustable gas system, this configuration prioritizes compact dimensions while retaining the .338-caliber capability of 338 ARC.
Each 338 ARC AR Upper can be configured through the product options. Customers can select the Cerakote finish, B5 rail-cover color, muzzle device, and whether to include the CM15 Low-Mass Bolt Carrier Group and CM15 Warhammer Ambidextrous Charging Handle.
Lightweight CM Billet Upper Receiver
The CM15 uses a lightweight upper receiver CNC machined from 6061 billet aluminum. Machined reliefs are incorporated throughout the receiver to remove unnecessary material, including a milled channel through the Picatinny rail.
- CM lightweight billet upper receiver
- 6061 aluminum construction
- Machined weight-reduction reliefs throughout the receiver
- Milled channel through the Picatinny rail
- Customer-selectable Cerakote finish
8.5″ 338 ARC Barrel with 1:5 Twist
At the center of the 338 ARC AR Upper is an 8.5-inch 416R stainless-steel barrel with a 1:5 twist. The barrel uses a medium profile, QPQ corrosion-resistant finish, pistol-length gas system, .750-inch gas journal, and 5/8×24 threaded muzzle.
Chattahoochee Munitions uses the faster 1:5 twist in its short-barrel 338 ARC configurations based on its experience manufacturing and testing heavy .338-caliber systems. In CM testing, the faster twist has performed particularly well with heavy projectiles such as the 307-grain Hornady Sub-X compared with the standardized 1:8 twist configuration.
- 8.5″ barrel length
- 416R stainless steel
- Medium profile
- QPQ corrosion-resistant finish
- 1:5 twist
- Pistol-length gas system
- .750″ gas journal
- 5/8×24 threaded muzzle
Compact 338 ARC Configuration
The 8.5-inch 338 ARC AR Upper is the most compact upper in the current CM15 338 ARC family. It is intended for customers who prioritize minimum barrel length and reduced overall dimensions over the additional barrel length offered by the 10.5-inch and 16-inch configurations.
Chattahoochee Munitions has observed powder utilization occurring around the 12-inch range with relevant 338 ARC loads evaluated by CM. The 8.5-inch configuration intentionally operates below that point in exchange for maximum compactness within the CM15 upper family. Actual velocity and performance depend on the specific ammunition and load.
Superlative Arms Adjustable Gas System
The CM15 338 ARC AR Upper uses a Superlative Arms adjustable gas block paired with a CM Melonite-treated pistol-length gas tube. The adjustable gas system provides a tuning point for different ammunition, operating conditions, and suppressed or unsuppressed configurations.
- Superlative Arms adjustable gas block
- .750″ gas journal
- Pistol-length gas system
- CM Melonite gas tube
CM 7.25″ M-LOK Handguard
The 8.5-inch 338 ARC AR Upper uses a compact 7.25-inch CM handguard manufactured from 6061 aluminum. The handguard provides M-LOK mounting capability, a full-length Picatinny top rail, and integrated QD sling sockets at the 3 and 9 o’clock positions.
- CM 6061 aluminum construction
- 7.25″ handguard length
- M-LOK mounting system
- Full-length Picatinny top rail
- QD sling sockets at 3 and 9 o’clock
- Customer-selectable Cerakote finish
- Customer-selectable B5 rail-cover color
Optional CM15 Low-Mass Bolt Carrier Group
Customers can order the 338 ARC AR Upper with or without the CM15 Low-Mass Bolt Carrier Group. This gives customers who already have compatible components the option to purchase the upper without duplicating the BCG.
When selected, the CM15 Low-Mass Bolt Carrier Group uses a 9310 hardened steel bolt with the requisite 6.5 Grendel Type II / .449″ bolt face for the 338 ARC system.
- 9310 hardened steel bolt
- 6.5 Grendel Type II / .449″ bolt face
- 17-4 stainless-steel firing pin
- 8620 carrier and key
- MPI tested
- Hardened S7 steel extractor
Optional CM15 Ambidextrous Charging Handle
The 338 ARC AR Upper can also be configured with the CM15 Warhammer Ambidextrous Charging Handle in collaboration with Breek Arms. Customers who already have a compatible charging handle can order the upper without this component.
Selectable Muzzle Device
The 8.5-inch barrel features a 5/8×24 threaded muzzle. Customers can select from the compatible muzzle configurations available on the product page, including a thread protector or available suppressor-host muzzle devices.
This allows the 338 ARC AR Upper to be ordered around the customer’s intended suppressor mounting system rather than requiring replacement of a fixed muzzle device after purchase.
Configure Your CM15 Upper
Each 338 ARC AR Upper is offered as a configurable assembly. Instead of separating colors and component combinations into multiple product pages, customers can select the combination appropriate for their build directly on the product page.
- Cerakote finish: Customer-selectable color
- B5 rail covers: Customer-selectable color
- CM15 Low-Mass BCG: With or without
- CM15 Warhammer Ambidextrous Charging Handle: With or without
- Muzzle configuration: Available suppressor-host muzzle device or thread protector
338 ARC in the AR-15 Platform
The 338 ARC AR Upper allows the .338-caliber 338 ARC cartridge to operate within the AR-15 platform envelope. That gives customers access to a heavy-projectile .338-caliber system without moving to the larger AR-10 platform used for cartridges such as 8.6 Blackout.
For reliable feeding, Chattahoochee Munitions offers a dedicated 338 ARC AR15 Magazine vetted by CM for the cartridge.
Built Around CM’s .338-Caliber Experience
Chattahoochee Munitions has years of manufacturing and testing experience with heavy .338-caliber cartridges and suppressed systems. That experience informs the architecture of the CM15 338 ARC AR Upper, including the 1:5 twist barrel, adjustable gas system, requisite bolt-face geometry, and selectable suppressor-host muzzle configuration.
The 8.5-inch upper is not simply a generic AR-15 upper with a 338 ARC barrel installed. The barrel, gas system, receiver, handguard, and optional operating components are configured together as a compact CM15 338 ARC system.
CM15 8.5″ 338 ARC AR Upper Specifications
- Cartridge: 338 ARC
- Platform: AR-15 / CM15
- Product: Configurable upper assembly
- Upper Receiver: CM lightweight 6061 billet aluminum
- Receiver Features: Machined weight-reduction reliefs and milled Picatinny rail channel
- Finish: Customer-selectable Cerakote color
- Barrel Length: 8.5″
- Barrel Material: 416R stainless steel
- Barrel Profile: Medium
- Barrel Finish: QPQ corrosion-resistant finish
- Twist Rate: 1:5
- Gas System: Pistol length
- Gas Journal: .750″
- Gas Block: Superlative Arms adjustable
- Gas Tube: CM Melonite
- Muzzle Thread: 5/8×24
- Handguard: CM 7.25″ 6061 M-LOK
- Handguard Features: Full Picatinny top rail and QD sockets at 3 and 9 o’clock
- B5 Rail Covers: Customer-selectable color
- BCG: Optional CM15 Low-Mass Bolt Carrier Group
- BCG Bolt: 9310 hardened steel
- Bolt Face: 6.5 Grendel Type II / .449″
- Firing Pin: 17-4 stainless steel
- Carrier and Key: 8620 steel
- Extractor: Hardened S7 steel
- BCG Testing: MPI
- Charging Handle: Optional CM15 Warhammer Ambidextrous Charging Handle in collaboration with Breek Arms
- Muzzle Configuration: Customer-selectable compatible suppressor-host muzzle device or thread protector
Explore Containment Zone 338
Visit Containment Zone 338 to compare CM15 complete firearms, CM15 upper assemblies, CM7S bolt-action systems, barreled actions, magazines, and technical information about 338 ARC.
338 ARC AR Upper FAQ
What is included with the CM15 338 ARC AR Upper?
The base upper includes the CM lightweight billet receiver, 8.5-inch barrel, 7.25-inch handguard, adjustable gas system, gas tube, and selected muzzle configuration. The CM15 Low-Mass Bolt Carrier Group and CM15 Warhammer Ambidextrous Charging Handle are selectable options.
What twist rate does the 8.5-inch CM15 338 ARC AR Upper use?
The 8.5-inch 416R stainless-steel barrel uses a 1:5 twist. Chattahoochee Munitions uses this faster twist in its short 338 ARC configurations based on CM’s experience and testing with heavy .338-caliber projectiles.
Can I choose the color of the upper?
Yes. Cerakote finish and B5 rail-cover color are customer-selectable options. Available selections are shown on the product page.
Are the BCG and charging handle included?
They are selectable options. Customers can add the CM15 Low-Mass Bolt Carrier Group, the CM15 Warhammer Ambidextrous Charging Handle, both components, or order the upper without them.
Can I choose a suppressor-host muzzle device?
Yes. The 5/8×24 threaded barrel can be configured with an available compatible suppressor-host muzzle device or a thread protector.
Does the CM15 338 ARC AR Upper use adjustable gas?
Yes. The 8.5-inch upper uses a Superlative Arms adjustable gas block with a CM Melonite-treated pistol-length gas tube.
What bolt is supplied with the optional CM15 BCG?
The CM15 Low-Mass BCG uses a 9310 hardened steel bolt with the requisite 6.5 Grendel Type II / .449″ bolt face. The assembly also uses an 8620 carrier and key, 17-4 stainless-steel firing pin, and hardened S7 steel extractor.
Where can I find the compatible CM magazine?
Chattahoochee Munitions offers a dedicated 338 ARC AR15 Magazine vetted by CM for the cartridge.
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