A friend of the GEARs Crew has been interested in ARs chambered pistol calibers and I thought this review posted over at Thefirearmblog.com was worth the read.
The Mk9 upper is made of forged 7075-T6 aluminum, and is chambered in 9mm, with a 9″ barrel made of nitrided 4140 chrome-moly steel in a 1:10″ twist. It is a simple, pure blowback operating system. Those who plan to use suppressors or muzzle devices on the Mk9 should be advised that the barrel is threaded in 1/2″-36 – this is the case with almost all 9mm AR-style carbines, perhaps due to the significant risk that a user might accidentally install a .223 muzzle device with 1/2″x28 threads onto a 9mm upper, which could be dangerous. Accordingly, if you plan to use a suppressor, you may have to obtain a thread adapter (I recommend a 1/2″x36 to tri-lug adapter, if you can locate it) or get a new piston/sleeve for your suppressor if it has interchangeable pistons (and be sure to install a fixed barrel adapter if your suppressor has a Nielsen device).