Andre, the MK9 uses Colt pattern magazines, while the MkGs uses Glock double stack magazines. Have a great day!

Andre, as others have mentioned the only difference is what magazines they take. The Mk9 uses Colt pattern magazines and the MKGS uses Glock 17/19 style magazines. Personally I'd go with the MKGS cause Glock mags are cheaper and easy to find. Not sure if you know but Bidens atf made a rule about pistol braces. Any pistol with a brace is now considered a sbr/short barreled rifle and must be registered with the ATF by may 31st or the brace be removed, or add a 16 inch barrel. I noticed buds is still showing a few of them with pistol braces wich won't be included. Hope this helps.

Hi Andre, the CMMG MkGs was CMMG's first 9mm Radial Delayed Blowback™ that was brought to market. It was engineered to accept small frame Glock double stack magazines like those found in the Glock 17, 19 and 26. The availability of Glock magazines combined with the popularity of Glock 9mm pistols made for an easy choice to release the MkGs. The MK9 uses Colt pattern 9mm 32 round magazines for the CMMG Mk9 platform. The MkGs is more popular but I prefer the MK9. Cheers and stay safe!

They differ in the magazines they use and hence have slightly different lower receivers. Mk9 uses the Colt pattern of straight magazines while the MkGs models use Glock magazines.