300 Blackout Pistol Upper – CM15 5″ 1:5 Twist
The CM15 300 Blackout Pistol Upper is an ultra-compact AR-15 upper receiver manufactured by Chattahoochee Munitions around a five-inch, 1:5 twist barrel. Its micro-length gas system, adjustable gas block and four-inch handguard create the shortest conventional-handguard upper in the CM15 300 Blackout family.
This upper is intended for customers who deliberately prioritize minimum length, suppressed subsonic use and maneuverability. The five-inch barrel does not provide the powder utilization or velocity available from CM’s preferred 10- to 10.5-inch general-purpose barrel range. Its defining advantage is compactness.
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.
Ultra-Compact 300 Blackout Pistol Upper
The 300 Blackout Pistol Upper combines a five-inch barrel with a four-inch conventional handguard. The handguard terminates behind the muzzle, leaving the thread protector, muzzle device or suppressor-mounting interface accessible.
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 prioritizing minimum upper-receiver length
CM Lightweight Billet Upper Receiver
The CM upper receiver is machined from 6061 billet aluminum and incorporates material reliefs to reduce unnecessary mass. The receiver 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
CM Four-Inch M-LOK Handguard
The CM 6061 aluminum four-inch handguard preserves the compact proportions of the 300 Blackout Pistol Upper while covering the barrel nut and micro-length gas system.
- 6061 aluminum construction
- Four-inch handguard length
- Free-floating configuration
- Full Picatinny top rail
- M-LOK accessory-mounting surfaces
- QD sling sockets at the 3 and 9 o’clock positions
- Conventional profile with the muzzle forward of the handguard
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 of the firearm and support manipulation around optics and other receiver-mounted equipment.
Micro-Length Adjustable Gas System
The 300 Blackout Pistol Upper uses a micro-length gas system because the five-inch barrel is too short to accommodate conventional pistol-length gas. This is a dimensional requirement specific to the five-inch configuration, not a recommendation for longer 300 Blackout barrels.
- Melonite-treated micro-length gas tube
- Superlative Arms .750-inch adjustable gas block
- Five-inch-specific micro gas architecture
- Adjustable gas control for the completed 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 micro-length gas architecture for the five-inch barrel. Final gas and buffer tuning must be completed on the assembled firearm because operation depends on the lower receiver, buffer configuration, ammunition, muzzle device and suppressor.
Explore the 300 Blackout gas-system and tuning guide for detailed information about gas adjustment, buffer adjustment, ejection patterns and single-round testing.
Five-Inch 1:5 Twist Barrel
- Five-inch barrel length
- 4140 GBQ steel
- Medium profile
- Nitride finish
- 1:5 twist rate
- Shouldered crown
- 5/8×24 muzzle threads
- .750-inch gas-block journal
- Micro-length gas system
The fast 1:5 twist is essential to the purpose of this short barrel. It provides the rotational stability needed for long, heavy subsonic bullets despite the extremely limited barrel length.
A five-inch barrel does not provide the same powder utilization as a 10- or 10.5-inch barrel. Customers selecting this 300 Blackout Pistol Upper should do so because minimum length is more important to their application than maximum velocity.
Muzzle-Device Options
The 5/8×24 threaded muzzle can be configured with a thread protector or a 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 five-inch barrel. A suppressor’s caliber rating alone does not establish compatibility with an extremely short barrel.
The four-inch conventional handguard leaves the suppressor and mounting system forward of the handguard. Customers wanting a suppressor to extend beneath the handguard should consider the dedicated CM15 five-inch SD firearm rather than this conventional upper.
Suppressed Subsonic Operation
The 300 Blackout Pistol Upper can use both subsonic and supersonic ammunition, but its primary 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 300 Blackout Pistol Upper
- Caliber: 300 AAC Blackout / 300 BLK
- Receiver: CM lightweight 6061 billet upper
- Barrel: Five-inch 4140 GBQ steel, nitrided
- Barrel profile: Medium
- Twist rate: 1:5
- Gas system: Micro length
- Gas block: Superlative Arms .750-inch adjustable
- Muzzle thread: 5/8×24
- Handguard: CM four-inch 6061 aluminum 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 300 Blackout Pistol Upper occupies the shortest conventional-handguard upper position within the CM15 family. Customers seeking greater powder utilization should consider a 7.5- or 10.5-inch CM15 upper.
Explore Sector 300 to compare complete firearms, uppers, CM7S bolt actions, magazines and technical 300 Blackout resources.
Frequently Asked Questions
What is a 300 Blackout Pistol Upper?
A 300 Blackout Pistol Upper is a short-barreled AR-15 upper receiver chambered in 300 Blackout and commonly used on a legally configured pistol lower. The same upper may also be used on a lawfully registered SBR.
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 micro-length gas?
The five-inch barrel is too short to accommodate conventional pistol-length gas. The micro-length system positions the gas port where sufficient operating pressure remains available to cycle the completed firearm.
Why does the barrel use a 1:5 twist?
The fast 1:5 twist provides the rotational stability needed for long, heavy subsonic bullets fired through the extremely short barrel.
Is five inches the optimal barrel length for 300 Blackout?
No. CM considers 10 to 10.5 inches optimal for general-purpose 300 Blackout performance. This five-inch upper is intended for customers who prioritize minimum length over maximum powder utilization and velocity.
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 five-inch barrel. The suppressor remains forward of the four-inch handguard.
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 takes place on the assembled firearm because the lower receiver, buffer system, ammunition and suppressor all affect operation. Begin with gas-block adjustment and use buffer adjustment for final refinement when available.
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