Here's a list of the complete Halo 3 weapons. It will take me a while to post it all but I'll get it.
Assault Rifle
Magazine Size: 32
Dual-Wieldable: No
This stripped-down starting weapon reduces your dependency on dual wield weapons, making Halo 3 feel closer to the first Halo. The AR is a big improvement over Halo 1's Assault Rifle, though. It inflicts massive damage at medium range and nearly guarantees you a kill. Still, its 32 round clip is not sufficient for battles against 2 or more enemies. Dubbed by some as "The Ultimate Spawning Weapon," this gas operated, fully automatic projectile rifle is as reliable as default weapons get, and against one unsuspecting enemy, the AR is all you need to debone their carcass with hot lead.
Pros: Extremely effective in slaying enemies for a spawning weapon, the UNSC Assault Rifle is also effective during Halo 3 multiplayer.
Cons: Shooting enemies to death with it takes many bullets, and against some of the stronger enemies in Halo 3's campaign, the Assault Rifle is less than an ideal choice to take them on.
Battle Rifle
Magazine Size: 36
Dual-Wieldable: No
Once you've mastered the Assault Rifle, your next weapon of choice may turn out to be the Battle Rifle, a deadlier version of the default gun. The Battle Rifle fires short, but potentially overwhelming, bursts of ammunition that can massacre your foes in just about any scenario. Zooming in on enemy targets with this gun should always be implemented when carrying this destructive weapon.
Pros: The Battle Rifle is especially useful for Spartans who have developed their aiming skills. Not only does it hold more rounds than the regular Assault Rifle, the Battle Rifle is much more devastating.
Cons: You have to be good at aiming to take full of advantage of this powerful gun.
Fire Bomb Grenades
Magazine Size: N/A - (You can hold 2 of each type of grenade at once)
Dual-Wieldable: No
The Fire Bomb Grenades are just what you'd expect incendiary grenades would be, grenades that explodes into a fiery blast that torches your targets. These grenades emit an almost napalm-like gel that incinerates anyone unlucky enough to be nearby when it detonates.
Pros: The blast from the Fire Bomb Grenade will instantly end the life of anyone standing within its 7 foot blast radius. Just pitch it into a group of enemies and let them burn!
Cons: You have to be accurate when tossing the Fire Bomb Grenades at enemies as its blast radius is relatively small.