CM15 338 ARC 10.5″ Upper
- CM 6061 lightweight billet aluminum receiver
- Reliefs featured throughout the receiver
- Picatinny rail milled channel for a sleek/light weight option
- CM 6061 9.5″ MLOK handguard
- QD Sockets at 3 and 9 o’clock
- Full Picatinny top rail
- Choice of Suppressor Host Muzzle Device or Thread Protector
- CM15 Low Mass Bolt Carrier Group
- 9310 Hardened Steel Bolt
- 17-4 Stainless Steel Firing Pin
- 8620 Carrier and Key
- MPI Tested
- Hardened Extractor S7 Steel
- CM15 Warhammer Ambidextrous Charging Handle – in collaboration with Breek Arms
- Superlative Arms Adjustable Gas Block
- CM Melonite Gas Tube
- CM15 338 ARC 10.5″ Barrel
- 416R Stainless Steel
- Medium Profile
- QPQ Corrosion Resistant
- 1:5 Twist
- Pistol Gas
- .750 Gas Journal
- 5/8×24 threaded muzzle
CM15 10.5″ 338 ARC Upper: The Ultimate Subsonic Sledgehammer
Step into the next evolution of big-bore performance with the CM15 338 ARC 10.5″ Upper. Purpose-built for the revolutionary 338 Advanced Rifle Cartridge, this upper receiver group is designed for shooters who want to bridge the gap between standard AR-15 maneuverability and the devastating terminal ballistics of .338 caliber projectiles.
Unmatched Stabilization with 1:5 Twist
The 338 ARC shines when pushing heavy, high-BC bullets, particularly in subsonic applications. To handle these massive projectiles, our 10.5-inch barrel features an ultra-fast 1:5 twist rate. This aggressive rifling ensures that heavy-grain bullets are perfectly stabilized the moment they leave the muzzle, providing surgical precision and preventing keyholing—even when running a suppressor.
Compact, Reliable, and Versatile
The 10.5″ barrel length offers the perfect “short-barrel rifle” (SBR) or pistol profile, making it an ideal choice for brush hunting, vehicle operations, or home defense. Built on a foundation of 7075-T6 aluminum, the upper features an enlarged ejection port to ensure the wider .338 brass cycles with flawless reliability.
Product Features:
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Caliber: 338 ARC (Optimized for heavy .338 projectiles).
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10.5” Heavy Barrel: 4150 CMV steel with a 1:5 twist for maximum stability.
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Tailored Options: Customize your build with selectable M-LOK handguards, high-performance BCGs, and specialized muzzle devices.
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AR-15 Compatible: Fits standard lowers; utilizes 6.5 Grendel-style magazines.
Whether you are building a dedicated suppressed “thumper” or a compact hunter, the CM15 338 ARC 10.5″ Upper delivers heavy-hitting authority in a lightweight, modern package.
Why Choose the Chattahoochee Munitions 338 ARC Upper?
When you need maximum energy transfer in a compact AR-15 footprint, standard calibers often fall short. The 338 ARC Upper solves this challenge by pairing modern cartridge engineering with precision firearm craft. Engineered specifically around Hornady’s groundbreaking .338 Advanced Rifle Cartridge, this upper receiver group delivers heavy-caliber kinetic impact without forcing you to transition to a heavier, bulkier AR-10 platform.
Whether you’re engaging targets through dense brush, building a whisper-quiet suppressed tactical rig, or upgrading your short-barrel setup, our 338 ARC Upper offers an unbeatable balance of power, maneuverability, and component reliability.
Engineered for Premium Suppressed Performance
Running big-bore subsonic rounds requires exact gas management and bullet rotation. The 338 ARC Upper is built from the ground up to excel under suppression:
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Ultra-Fast 1:5 Twist Barrel: Heavy subsonic .338 projectiles require significant rotational velocity to remain stable. Our 1:5 fast twist barrel locks in flight stability immediately upon ignition, eliminating projectile tumbling and protecting your suppressor baffles from devastating baffle strikes.
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Superlative Arms Adjustable Gas Block: Fine-tune gas flow to match your exact ammunition load and silencer setup. The patented bleed-off technology vents excess gas forward away from the shooter, reducing blowback, mitigating dirty receiver fouling, and softening perceived recoil.
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CM15 Low-Mass Bolt Carrier Group: Crafted with a 9310 hardened steel bolt and S7 steel extractor, our custom BCG handles higher case rim pressures smoothly while maintaining cycling speed.
Key Performance Specifications
Customization Options to Match Your Mission
Every shooter’s needs are unique. That is why each 338 ARC Upper is built with customizable factory options:
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Custom Cerakote Finishes: Choose from premium color coats including Armor Black, Flat Dark Earth (FDE), OD Green, Tungsten, Sniper Grey, Burnt Bronze, Cobalt Kinetics Green, and FDE Metallic.
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Suppressor-Ready Muzzle Devices: Select your preferred mount directly from the factory, including the SilencerCo ASR, Q Cherry Bomb, or Dead Air Xeno, or opt for a streamlined thread protector.
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Complete or Stripped Upper Setup: Order your 338 ARC Upper fully complete with our Low Mass BCG and Warhammer Ambidextrous Charging Handle, or select the bare upper configuration if you already have custom components ready.
Balancing AR-15 Portability with Big-Bore Energy
For decades, getting .338-caliber impact meant carrying heavy AR-10 frames or custom bolt actions. The 338 ARC Upper changes that paradigm by dropping directly onto any standard Mil-Spec AR-15 lower receiver.
By retaining standard AR-15 controls, trigger options, and lower receivers, you maintain lightweight handling while firing projectiles that deliver double the downrange kinetic energy of traditional .223/5.56 NATO loads. Combined with 6.5 Grendel/6mm ARC pattern magazines, feeding remains consistent whether firing supersonic hunting loads or whisper-quiet heavy subsonics.
Helpful Links & Related Products
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Complete Your Build: Pair your upper with our custom CM15 Lower Receivers for seamless aesthetic and lock-up fitment.
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Explore Caliber Options: Compare performance across our range of big-bore options including the CM15 8.6 Blackout Upper and CM15 300 BLK Upper.
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Ammunition & Suppressors: View our recommended host suppressors at the official Silencer Shop Network or explore cartridge technical specs directly via Hornady Manufacturing.
Frequently Asked Questions (FAQ)
1. What lower receiver do I need for a 338 ARC Upper?
The 338 ARC Upper is engineered to fit standard AR-15 lower receivers. You do not need a larger AR-10/LR-308 frame. Simply snap this complete upper group onto your existing AR-15 lower, insert a compatible magazine, and you are ready to shoot.
2. Which magazines work with the 338 ARC Upper?
Because the 338 ARC cartridge shares its parent case geometry with the 6.5 Grendel family, it feeds reliably from standard 6.5 Grendel or 6mm ARC pattern AR-15 magazines.
3. Why is a 1:5 twist rate important for a 10.5″ 338 ARC Upper?
The 1:5 fast twist rate is critical for stabilizing heavy .338 projectiles out of shorter barrels. Heavy subsonic bullets (often 250 to 300+ grains) require rapid spinning to prevent keyholing in flight and to avoid baffle strikes when shooting through a silencer.
4. Do I need an adjustable gas block for this upper?
Yes, and we include the Superlative Arms Adjustable Gas Block standard. Because ammunition velocity and suppressor backpressure vary significantly, an adjustable gas block lets you tune gas pressure for soft shooting, zero blowback, and 100% cycling reliability.
5. Is the ejection port modified on this upper?
Yes. Standard AR-15 upper receivers have smaller ejection ports designed for .223 cases. Our 338 ARC Upper features a factory-enlarged ejection port to ensure large-diameter .338 brass ejects cleanly without casing hangs or stovepipes.
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