DPM Systems AR-15 9mm Buffer Spring Kit

Fun fact:  AR manufacturers make 9mm direct blowback, pistol caliber carbines (PCCs) are made for one good reason:  It's cheap!  That's the same reason why beer companies sold you on the lie of hard seltzer!  

The problem with that is 9mm ARs have a ton of recoil and the only way to soften that is heavier and heavier buffers, springs.  Or you can try this DPM spring kit which uses a hydraulic buffer, and springs to help soften the recoil.