M443 Caseless Full Metal Jacket
From Halopedia, the Halo wiki
The M443 5x23mm Caseless FMJ is a type of ammunition used by the UNSC in their M7 Caseless Submachine Gun. It is fired from a 60-round magazine locaded on the left side of the SMG. Because it is caseless ammunition, there is no need for an ejector, as there are no "spent" shell casings.