Super Carbine
From Halopedia, the Halo wiki
The Super Carbine is a Carbine found on the bodies of dead Brutes in certain Arbiter levels. Unlike its regular counterparts it contains 120 rounds instead of the standard 72. Unfortunately, this heightened ammo capacity is only temporary, for it is unable to be replenished to 72 rounds once you have emptied the "Super" clip.
Also, on the level Gravemind, if you take one off the carbine-wielding marine, you get 120 ammo.
