CM15 300BLK 7.5″ 1:5 Twist Upper with Options

Compact 300 Blackout Upper – CM15 7.5″ 1:5 Twist

The CM15 Compact 300 Blackout Upper is a short, maneuverable AR-15 upper receiver manufactured by Chattahoochee Munitions around a 7.5-inch, 1:5 twist barrel. Its pistol-length gas system, adjustable gas block and SLR Rifleworks 6.7-inch handguard create a compact conventional configuration with the muzzle and suppressor interface forward of the handguard.

This upper is intended for customers who prioritize reduced length, suppressed subsonic use and close-range handling. The 7.5-inch barrel is more compact than CM’s preferred 10- to 10.5-inch general-purpose 300 Blackout barrel range, but it provides more barrel length and powder utilization than the specialized five-inch upper.

The upper can support either a pistol or lawfully registered SBR configuration depending on the lower receiver, stock or brace arrangement and applicable law. An upper receiver does not independently establish the legal classification of the complete firearm.

Purposeful Compact 300 Blackout Upper

The Compact 300 Blackout Upper combines a 7.5-inch barrel with a 6.7-inch conventional handguard. This leaves the muzzle device and suppressor mounting interface accessible while controlling the overall length of the completed firearm.

This configuration is particularly well suited to:

• Compact suppressed subsonic shooting
• AR-15 pistol configurations
• Lawfully registered short-barreled rifles
• Close-range training and range use
• Confined-space handling
• Customers wanting more barrel length than a five-inch upper

CM Lightweight Billet Upper Receiver

The CM upper receiver is machined from 6061 billet aluminum and incorporates material reliefs to reduce unnecessary mass. It is manufactured as part of the complete upper system rather than treated as an unrelated collection of components.

• 6061 billet aluminum construction
• Lightweight receiver profile
• Machined reliefs throughout the receiver
• Channeled Picatinny rail for reduced mass and a streamlined profile
• Compatible with standard AR-15-pattern lower receivers

SLR Rifleworks ZRO 6.7-Inch Handguard

The Compact 300 Blackout Upper uses an SLR Rifleworks ZRO 6.7-inch M-LOK handguard. Its short length maintains muzzle-device and suppressor-mount access while preserving useful accessory space.

• 6.7-inch exact length
• 1.35-inch internal diameter
• Free-floating construction
• Mil-spec hardcoat-anodized finish
• Hardcoat-anodized 7075 aluminum barrel nut
• No barrel-nut timing or shims required
• Slip-fit locking attachment system
• Two limited-rotation QD sling sockets
• Continuous Picatinny top rail
• M-LOK accessory-mounting surfaces
• Useful forward mounting space for compatible night-vision and infrared equipment

This is a conventional handguard rather than an SD handguard. A suppressor remains forward of the handguard rather than extending beneath it.

CM15 Low-Mass Bolt Carrier Group

The included CM15 low-mass bolt carrier group reduces reciprocating mass and works with the adjustable gas system to support controlled operation once the completed firearm has been properly tuned.

• 9310 hardened-steel bolt
• 8620 steel carrier and gas key
• 17-4 stainless-steel firing pin
• Magnetic-particle-inspected bolt
• Hardened S7 tool-steel extractor
• Low-mass carrier configuration

Ambidextrous Charging Handle

The upper includes a CM15 Warhammer ambidextrous charging handle produced in collaboration with Breek Arms. Its ambidextrous controls provide access from either side and support manipulation around optics and other receiver-mounted equipment.

Adjustable Pistol-Length Gas System

The Compact 300 Blackout Upper uses a pistol-length gas system. Unlike CM’s five-inch upper, which requires micro gas because of its limited barrel length, the 7.5-inch barrel accommodates the pistol-length gas architecture appropriate for this configuration.

• Melonite-treated pistol-length gas tube
• Superlative Arms .750-inch adjustable gas block
• Pistol-length gas architecture
• Adjustable gas control for final tuning on the assembled firearm

An adjustable gas block is a requisite component on a properly configured semi-automatic 300 Blackout system. Subsonic ammunition, supersonic ammunition and suppressors can create substantially different operating conditions.

The gas block addresses the root cause by controlling how much gas enters the operating system. Buffer adjustment, when available on the completed firearm, should be used for final refinement after the gas setting has been established.

The upper is manufactured with the correct pistol-length gas architecture, barrel gas-port specification and adjustable gas components. It is not pre-tuned for an unknown lower receiver. Final gas and buffer tuning must be completed on the assembled firearm because the lower receiver, buffer weight, buffer spring, ammunition and suppressor all influence cycling.

Explore the 300 Blackout gas-system and tuning guide for detailed information about gas adjustment, buffer adjustment, ejection patterns and single-round testing.

7.5-Inch 1:5 Twist Barrel

• 7.5-inch barrel length
• 4150 CMV steel
• Medium profile
• Nitride finish
• 1:5 twist rate
• Shouldered crown
• 5/8×24 muzzle threads
• .750-inch gas-block journal
• Pistol-length gas system

The fast 1:5 twist provides greater rotational stability for long, heavy subsonic bullets than the former 1:7 configuration. This makes the revised barrel substantially better aligned with the suppressed and subsonic purpose of 300 Blackout.

The 7.5-inch barrel does not provide the same powder utilization as CM’s preferred 10- to 10.5-inch barrel range. Customers selecting this Compact 300 Blackout Upper should do so because reduced length is more important to their application than maximum ballistic efficiency.

Muzzle-Device Options

The 5/8×24 threaded muzzle can be configured with a thread protector or compatible suppressor-host muzzle device. Available options may include direct-thread interfaces, flash hiders, brakes and suppressor mounts. Pricing may vary according to the selected device.

Any suppressor used with this upper must be rated by its manufacturer for 300 Blackout and a 7.5-inch barrel. A suppressor’s caliber rating alone does not establish short-barrel compatibility.

The 6.7-inch conventional handguard leaves the suppressor and mounting system forward of the handguard. Customers wanting a suppressor recessed beneath an extended handguard should consider the dedicated CM15 7.5-inch SD firearm.

Suppressed Subsonic Operation

The Compact 300 Blackout Upper can use both subsonic and supersonic ammunition, but its principal purpose is compact suppressed operation with subsonic loads.

For subsonic ammunition, an enclosed suppressor is generally preferable to a low-backpressure or flow-through design. Subsonic loads often benefit from the additional backpressure supplied by an enclosed suppressor. A flow-oriented design can reduce the operating energy available to cycle the firearm.

Supersonic ammunition used with an enclosed suppressor can generate considerably more backpressure. The adjustable gas block allows incoming gas to be reduced rather than attempting to mask excessive gas with a heavier buffer alone.

Suppressor choice, ammunition, gas setting and the completed firearm’s buffer system function together. Even changing between different subsonic loads can affect cycling and ejection behavior.

Black Butterfly Ammunition is an appropriate source for purpose-designed 300 Blackout ammunition.

Explore the 300 Blackout subsonic guide for additional information about barrel length, twist rate, ammunition and suppression.

Complete Upper Specifications

• Manufacturer: Chattahoochee Munitions
• Product: CM15 Compact 300 Blackout Upper
• Caliber: 300 AAC Blackout / 300 BLK
• Receiver: CM lightweight 6061 billet upper
• Barrel: 7.5-inch 4150 CMV steel, nitrided
• Barrel profile: Medium
• Twist rate: 1:5
• Gas system: Pistol length
• Gas block: Superlative Arms .750-inch adjustable
• Muzzle thread: 5/8×24
• Handguard: SLR Rifleworks ZRO 6.7-inch M-LOK
• BCG: CM15 low-mass bolt carrier group
• Charging handle: CM15 Warhammer ambidextrous
• Muzzle device: Selectable suppressor host or thread protector

Installation and Legal Classification

The purchaser is responsible for ensuring the upper is installed on a compatible lower receiver and that the resulting firearm complies with all federal, state and local requirements.

Installing a barrel shorter than 16 inches on a lower receiver equipped with a shoulder stock generally creates a short-barreled rifle subject to the National Firearms Act unless a lawful exception applies. Customers should determine the applicable requirements before assembling or reconfiguring a firearm.

Explore Sector 300

This Compact 300 Blackout Upper occupies the middle position between CM’s ultra-short five-inch upper and its more ballistically efficient 10.5-inch upper. It provides a compact conventional-handguard configuration without placing the suppressor beneath the handguard.

Explore Sector 300 to compare complete firearms, uppers, CM7S bolt actions, magazines and technical 300 Blackout resources.

Frequently Asked Questions

What is a Compact 300 Blackout Upper?

A Compact 300 Blackout Upper is a short AR-15 upper receiver chambered in 300 Blackout. This CM15 uses a 7.5-inch barrel, pistol-length gas system and 6.7-inch conventional handguard.

Does the upper determine whether the firearm is a pistol or SBR?

No. Classification depends on the lower receiver, stock or brace configuration, barrel length and applicable law. The upper alone does not determine the complete firearm’s classification.

Why does this upper use pistol-length gas?

The 7.5-inch barrel accommodates pistol-length gas, which provides the appropriate gas architecture for subsonic and supersonic 300 Blackout operation. Micro gas is reserved for CM’s shorter five-inch configurations.

Why does the barrel use a 1:5 twist?

The fast 1:5 twist provides greater rotational stability for long, heavy subsonic bullets fired through the short barrel.

Is 7.5 inches the optimal barrel length for 300 Blackout?

No. CM considers 10 to 10.5 inches optimal for general-purpose performance. The 7.5-inch upper is intended for customers who prioritize reduced length and maneuverability.

Does the upper include a BCG and charging handle?

Yes. It includes the CM15 low-mass bolt carrier group and CM15 Warhammer ambidextrous charging handle unless a product option explicitly states otherwise.

Can a suppressor be installed?

Yes. The muzzle is threaded 5/8×24, but the suppressor must be rated for 300 Blackout and a 7.5-inch barrel. The suppressor remains forward of the handguard.

Is this an SD upper?

No. This upper uses a conventional 6.7-inch handguard. A suppressor does not extend beneath it.

Can it use subsonic and supersonic ammunition?

Yes. Different ammunition and suppressors can alter cycling and ejection behavior, so gas and buffer tuning may be required when the completed firearm’s operating configuration changes.

Does the upper require final tuning?

Yes. Final tuning must occur on the assembled firearm because the customer’s lower receiver, buffer system, ammunition and suppressor all affect operation.