From Rap Dictionary
>===noun=== hollow tips
Hollow tips are a round of ammunition that are hollow at the end. Unlike regular bullets, the end crumples or tears off and there's a bigger surface area that causes more damage. Thirty-two shots in the clip, hollow tips in the Mack -- G-Unit (Gangsta Shit) 
Mack is used in the wrong term here. How can you have bullets in a person that spits game? It's Mac, which is short for Mac-Ingram, which is a type of gun.