Shop › CMMG 10RND EXT FN5.7 FITS 20RD MAGS CMMG 10RND EXT FN5.7 FITS 20RD MAGS $17.95 CMMG Magazine Extension Kit, Fits FN 5.7x28 Pistol, Adds 10 Rounds, Black 57AFD1E Quantity: Add To Cart Added! CMMG Magazine Extension Kit, Fits FN 5.7x28 Pistol, Adds 10 Rounds, Black 57AFD1E