I dont think rapid reload will help you in Halo3. Rapid reload exploits the animation sequence in COD by quickly changing guns (Y-Y) after the clip is inserted to the gun, and before the gun is caulked. I think this was very intentional by the devolopers as it assumes that when switching guns the caulking of the mechanism takes place while raising the gun to its useful tactical postion, thus no animation nessecary. So as long as the clip is in, the caulking can take place in animation, or while switching weapons.
In Halo the developers would of also had to pay attention to this type of detail, not to mention actually having a caulking animation in their fictitious weapons. (there may be just havent paid attention to it)
All in all rapid reload only applies to COD in order to exploit the attention to detail that the developers put into the game. If you know of a way to shave time off Halo3 reloads, let me know and I will wip a chip up for you.
I know caulking is not the correct spelling, the correct spelling has a censored word in it