For CMMG Guard 9mm guns with the Radial Delayed Blowback operating system and a collapsible stock, you need a buffer that is the same length as a standard AR-15 carbine buffer (3.25”). Our 9mm hydraulic buffers, which are 4.0” in length and 6.0 oz in weight are normally longer and heavier than needed for this application. The best choice is our 3.25” KynSHOT hydraulic buffer for AR-15 carbines “High Pressure” (Part# RB5000HP), which is slightly heavier than the standard model at 4.8 oz and works very well for this application. We recommend pairing it with a SPRINCO Extra Power AR-15 carbine buffer spring (color code RED – Part# SPRINCO-25004).