The Ultimate PCC AR Build

Introduction

In this build guide, Caleb from Brownells introduces what he considers the ultimate pistol caliber carbine (PCC) AR-15 build. Unlike traditional 9mm AR-15s that use direct blowback systems, this setup is based on the CMMG radial delayed blowback design, which reduces recoil, allows for lighter components, and delivers a smoother shooting experience.

Radial Delayed Blowback vs Direct Blowback

Most 9mm AR-15 PCC builds rely on direct blowback operation. This system uses the energy from the cartridge to push the bolt straight back, which requires heavier bolts and buffers to keep the gun in battery. The result is more reciprocating mass and noticeable recoil.

By contrast, the radial delayed blowback system uses angled locking lugs on the bolt and barrel extension. These lugs create rotational friction during unlocking, delaying the cycle just enough to reduce energy transfer. This allows for lighter bolts and standard AR-15 buffer weights, making the gun feel closer to a standard 5.56 AR in terms of recoil and control.

CMMG Radial Delayed Bolt and Barrel Design

The heart of this build is the CMMG radial delayed blowback bolt and barrel combo. While the bolt looks like a standard AR-15 bolt, it has angled lugs and a spring-loaded ejector built in. The barrel extension mirrors this design with matching chamfers that create the delay. This setup allows shooters to use lighter parts without sacrificing reliability.

Buffer and Recoil System Setup

For this build, Caleb uses the BCM MK2 recoil system with a lightweight T0 buffer. Unlike traditional PCCs that require extremely heavy buffers, the CMMG system works with standard AR-15 buffer weights, eliminating unnecessary mass while still maintaining control.

AR-15 Lower Parts and Magazines

The lower receiver is a standard LMT Mars AR-15 lower, proving that you do not need a proprietary PCC lower for this build. The key is using Mean Arms magazines, which replicate the size of standard AR-15 mags and fit perfectly into a standard magwell. These mags also engage the bolt catch, a feature missing from many PCC builds. They are compatible with standard chest rigs and pouches, making them a great choice for this setup.

Additional lower parts include:

• Magpul TR stock with quick-detach sling options

• Die Free Company Kung Fu grip

• Forward Controls Design Q lever safety

• Geissele SSA-X Lightning Bow trigger

• Forward Controls mag release

AR-15 Upper Receiver and Barrel Installation

The upper receiver is a standard forged milspec AR-15 upper. The CMMG 9mm barrel is installed using Brownells Action Lube for proper fit. Since this system does not require a gas tube, installation is simpler than a traditional AR-15 upper build.

Handguard and Barrel Nut Setup

The build uses a Midwest Industries SP handguard, designed to fit over a suppressor. The barrel nut is torqued down without the need to align for a gas tube, since this PCC is not gas-operated. This makes assembly faster and easier.

Installing the Muzzle Device

A Dead Air muzzle brake with Zeno mount is added to allow suppressor attachment. The 9mm barrel uses standard 1/2x28 threading, but shooters must use a dedicated 9mm muzzle device with the correct bore size.

Handguard and Suppressor Fitment

The handguard slides over the barrel and suppressor, creating a sleek and functional profile. Caleb recommends removing the suppressor when installing accessories to prevent scratching. Once complete, the suppressor tucks neatly under the handguard for a compact, suppressed PCC.

Optic and Accessory Setup

The optic of choice is a Sig Sauer Romeo 5 red dot, known for its reliability and long battery life. Accessories include rail covers for comfort and improved grip.

Installing Vertical Grip and Hand Stop

To ensure safe handling with the short barrel and suppressor, a Unity vertical grip and Die Free Company Full Stop hand stop are installed. These provide indexing points for the support hand and keep the shooter’s grip safely behind the suppressor.

Rail Covers for Comfort and Grip

Rubberized M-LOK rail covers are added to enhance grip and comfort. These also protect hands from heat during extended shooting sessions.

Charging Handle and BCG Installation

A Radian Raptor ambidextrous charging handle (Sledgehammer variant) is installed along with the CMMG radial delayed bolt carrier group. The components are lubricated for smooth cycling and reliability.

Upper and Lower Assembly

The upper and lower receivers are joined, and the suppressor is mounted. The result is a compact, reliable, and smooth-shooting 9mm AR-15 PCC build that functions with standard AR parts wherever possible.

Final Thoughts and Wrap Up

This CMMG radial delayed blowback PCC build demonstrates the advantages of moving beyond traditional direct blowback systems. With reduced recoil, lighter parts, and compatibility with standard AR-15 components, it is a versatile and effective setup for range use, competition, or training.

Caleb closes by inviting shooters to share their own PCC builds and encourages anyone with questions to contact the Brownells Tech Line for support.