I remember playing Halo 2 on Xbox, in one of the sections where you control the Arbiter an are fighting the Flood. They drop all kinds of things, interstingly enough. From grenades (if you're lucky) to the occasional Shotgun or Battle Rifle. Interesting tip: you can let a good dozen or so Flood Infection forms pop on your shields before you're in trouble. Maybe it's less in Halo 1, but it saves you ammo.
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Squad Leader, 58th Regt., 2nd Batt., "Morse's Marauders" - U.S.S. Azuera
Primary MOS: Rifleman
Secondary MOS: Combat Demolitions
"When the going gets tough, the tough call on the Colonial Marines."