The Mauler, BR, MA5C AR, Carbine, Beam Rifle, SMG, Sentinel Beam, Missile Pod, Flamethrower, Spike grenades, and Firebomb grenades are all Halo 3 weapons that aren't included in Halo: Reach. Also, the M6G Magnum returns for Halo: Reach, but it now features a low-power scope and a faster rate of fire, making for a completely different feel of the weapon between the games. Most the weapons listed here were replaced by new weapons in the game, while the Mauler, SMG, Flamethrower, Spike grenades, and Firebomb grenades were taken out with no real replacement...
- The BR was replaced by the M392 DMR.
- The MA5C AR was replaced by the MA37 AR.
- The Carbine was replaced by the Needle Rifle.
- The Beam Rifle and Sentinel Beam were combined into the Focus Rifle.
- The Missile Pod's tracking ability given was to the Rocket Launcher.
If you count the vehicles in Halo 3 as weapons, a few did not make an appearance in Halo: Reach...
- The Hornet and Hornet Transport gunships were essentially replaced by the Falcon.
- The Elephant did not appear in Halo: Reach.
- The Chopper and Prowler were both cut, but one could argue that they were combined into the Revenant.