USED CMMG FNH FiveseveN 10 Round Mag Extension Kit. I bought this just to try it out, but found out that you need to sand and debur the magazine interface to get this extension to run reliably and I didn't want to mess with that. The extender and spring were used for maybe two range outings, and look like new. I paid $25 new, am selling used for $10! I can meet in the Greater Seattle area, or ship for 2 bucks more. Cash or Paypal accepted. Happy New Year!