Understanding CMMG’s Mk9 Platform


The Mk9 has been a staple in CMMG’s line-up for years. Featuring a dedicated magwell designed to accept Colt SMG-style 9mm magazines, the Mk9 has always been one of CMMG’s most popular firearms.

Up until recently, the Mk9 utilized a straight blowback operating system, similar to most 9mm AR’s on the market.

CMMG has now upgraded the Mk9 to utilize the patented Radial Delayed Blowback™ operating system, available in BANSHEE™ and RESOLUTE configurations. Offering significantly less felt-recoil, Radial Delayed Blowback has proven to be superior to the old straight blowback design and this change completes CMMG’s transition to the new and better operating system.

The Mk9 BANSHEE is offered as a pistol or short barreled rifle with barrel lengths of 5 or 8 inches in length, while Mk9 RESOLUTE rifles come with a 16-inch barrel. Both the BANSHEE and RESOLUTE offer Last Round Bolt Hold Open.


Along with offering complete Mk9 firearms and lower receiver groups that are designed to utilize Radial Delayed Blowback, CMMG offers Mk9 Upgrade Kits for those who already have an older CMMG Mk9 Lower Receiver.

The Mk9 Upgrade Kits have all the parts needed to make an old Mk9 compatible with a CMMG 9mm Radial Delayed Blowback upper receiver, including a mag catch, bolt catch, carbine buffer assembly, carbine action spring and feed ramp.

It’s important to note that there are two different versions of the Mk9 Upgrade Kits. Depending on the types of magazines you plan to use, you will want to either select the ASC/CPD magazine version or the Colt/Metalform magazine version. The kit you select will include a feed ramp that is designed for optimal reliability with that style of magazine. If you have any questions on which Mk9 Upgrade Kit is right for you, please email techsupport@cmmg.com.


If you’re planning to purchase a Mk9 lower receiver group or if you are planning to upgrade your existing Mk9, it is important that you check whether your CMMG 9mm Upper Receiver will be compatible.

CMMG has made a small modification to its 9mm barrels in order for them to be compatible with Mk9 lower receivers. This modification only applies to Mk9 platform lower receivers and does not positively or negatively affect functionality of the barrel with a MkGs, Mk4 or Mk17 platform lower receiver.

Barrels that are stamped “9mm Luger” will be compatible with an upgraded Mk9 lower receiver. If your barrel is marked “9x19 NATO”, you will need to purchase a new 9mm barrel in order to ensure reliable function.



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