Infinite Grenade Launcher Glitch (read description) | Halo SPV3
This lasts for the entire level. Just refill your grenades right before the gravity lift section. While the text "Grenade Launcher ammo replenished" is still on screen, move up until the game gives you a checkpoint. Make sure you're holding onto the battle rifle the whole time.
(Yes, I went back and figured out how to do it again!)
This may work on other levels simply by manually saving immediately after replenishing grenade ammo, but I have not thoroughly tested that, yet.