Accepting Sig Sauer M17/P320 magazines, the CMMG Mk17 delivers a new level of performance to the PCC world. Not only is it on an elite playing field in terms of fit and function, the Mk17 brings out all the features you've come to expect from CMMG, including Last Round Bolt Hold Open. Designed to operate exclusively with our Radial Delayed Blowback upper, or build using our Barrel and Bolt combo.Lifetime Quality GuaranteeWe aim to please, that's why every CMMG product is backed by our Lifetime Quality Guarantee against material and workmanship defects. Learn More.**Mk17 Lowers are designed to work with CMMG 9mm Radial Delay Blowback Uppers only**