CMMG MK4 Resolute 16.1" 9mm, Sniper GreyDescription RESOLUTE, Mk4, 9mm, 16. 1" Not only is 9mm one of the most prolific calibers for handguns, 9mm also makes a great choice for your next pistol caliber carbine. Although this CMMG RESOLUTE uses the same 9mm Radial Delayed Blowback upper system as our other 9mm RESOLUTES and BANSHEES this Mk4 RESOLUTE is unique in the fact that it uses our 9mm AR Conversion magazines. Whats the benefit of using an AR15 lower receiver? By swapping the upper