From Halopedia, the Halo wiki
Ranged Weapon Switching is a skill that can be used to grab weapons from afar at the cost of a grenade or explosive device. However, the placement of the thrown grenade would determine the direction of the weapon which varies. This is very useful when you are in a dire need of another weapon to suit the situation, or if you need ammunition.
First, throw the grenade just behind your preferred weapon of choice. When the grenade explodes on impact and landing, the weapon will go flying towards you (if you aimed right). Jump as the weapon gets past you and hold the action button. If done correctly, you should acquire the weapon at a given fast amount of time. If you already have that weapon in your disposal, you will only receive the ammunition. However the grenade must be placed just right, otherwise there is a good chance the weapon simply ends up even farther from you.
Weapons to use