The AR-15 conversion adapter will fit any standard AR-15 556/223 lower and convert it to 9mm with no required modifications to your lower or magazines. This mag well adapter has a built in magazine release for your Glock mags, locks in place with your standard AR-15 magazine catch, releases the same way AR mags do and has a bolt hold open feature that locks your bolt back after the last round. This 9mm AR adapter accepts Glock style double stacked 9mm magazines and comes with one 10 round magazine. *You must install a complete 9mm AR Rifle or Pistol Upper receiver onto your 556/223/300blk lower receiver for this to fit and operate.