Shredder round
From Halopedia, the Halo wiki
Shredder rounds are designed to splinter on impact, tearing through light armor and rending flesh. They were usually fired from Assult Rifles as a replacement for Armour Piercing Rounds. However, they do have a severe lack of penetration power and they are all but useless against Covenant personnel shields.
This type of ammunition was often used by the Spartan-II's when they ran out of AP ammunition, for example in the Lambda Serpentis System in 2535, the Spartans of Blue Team switched to using shredder rounds after exhausting all of their AP rounds when gunning down a horde of Grunts.
