CMMG 9mm Replacement Barrel & BCG Kit - 5" 4140CM SBN (Model 99D17C3)

The CMMG 9mm Replacement Barrel & BCG Kit is a complete bolt carrier group and barrel assembly designed to let you convert or build a 9mm pistol caliber carbine with CMMG's proprietary Radial Delayed Blowback System. This 5-inch kit pairs a chrome moly barrel with a patent-pending RDB bolt carrier to deliver the reliability and controllability serious shooters want in a 9x19 NATO platform.

Unlike traditional direct blowback designs that can feel snappy and wear parts faster, CMMG's delayed blowback approach uses a rotating bolt that unlocks progressively—slowing the carrier's rearward travel. The result is noticeably less perceived recoil, a lighter bolt carrier overall, and a rifle that balances handling weight with shooter comfort. Built on the 4140 chrome moly steel with salt bath nitride treatment inside and out, this 5-inch barrel is tough enough for serious use and threaded 1/2-28 for suppressor mounting if you go that direction.

This kit is engineered specifically for CMMG MkG lower receivers, though it may work with other mil-spec uppers if they have an enlarged ejection port—a critical requirement for reliable function. At 1.4 pounds, the short barrel keeps your overall rifle trim and nimble, making it a smart choice for home defense, fun plinking, or backup rifle builds where 9mm ammo is cheaper and easier to source than centerfire rifle rounds.

If you're building or converting to 9mm PCC, this complete barrel and BCG assembly saves time and hassle compared to sourcing parts separately.

Key Features

• Radial Delayed Blowback System (patent pending) reduces felt recoil and extends component life
• 5-inch barrel in 4140 chrome moly with salt bath nitride finish for durability
• Threaded 1/2-28 for suppressor compatibility
• Lightweight carrier design reduces rifle weight and improves handling
• Complete bolt carrier group and barrel—no additional parts needed to function
• 9x19 NATO chamber for broad ammo compatibility
• Optimized for CMMG MkG lowers and mil-spec uppers with enlarged ejection port

Specifications

Frequently Asked Questions

Will this barrel and BCG kit work with my AR-15 lower?

This kit is designed exclusively for CMMG MkG lower receivers. It may function with other mil-spec uppers if they have an enlarged ejection port, but CMMG cannot guarantee reliability outside their system. We recommend checking compatibility with your specific lower before purchase.

What makes the Radial Delayed Blowback system different from direct blowback?

The RDB bolt is forced to rotate and unlock before the carrier cycles rearward, which slows the blowback impulse. This reduces felt recoil, decreases wear on internal parts, and allows for a lighter carrier—all without adding weight to the rifle itself. It's a more refined shooting experience than straight blowback at the same price point.

Can I use this with a suppressor?

The barrel is threaded 1/2-28, which is suppressor-compatible. However, consult CMMG's documentation on RDB performance with suppressed fire, as back-pressure can affect bolt timing in delayed blowback systems.

What's included in this kit?

This kit includes the complete 5-inch barrel assembly and the bolt carrier group. You will need a CMMG MkG lower receiver and a mil-spec upper receiver (with enlarged ejection port) to complete a functional rifle.