From Halopedia, the Halo wiki
Ranged Weapon Switching is a trick that can be used to grab weapons from afar at the cost of a grenade or explosive device. It is useful if the player is in a dire need of another weapon to suit the situation, or if they need ammunition as quickly as possible.
This can be performed either with a Frag Grenade or a plasma grenades, or with a Rocket Launcher. Grenades are considered the best choice, as a rocket can make the weapon fly faster and farther, making ranged weapon switching harder to perform.
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. Jump as the weapon gets past you and hold the action button. If done correctly, you should acquire the weapon at relatively small amount of time. If you already have that weapon in your disposal, you will only receive its ammunition. Explosive type and placement is vital to perform this trick properly, as you can easily send the desired weapon towards a wrong direction.
This trick is mostly useful in Halo: Combat Evolved and Halo 2. In Halo 3, weapons can no longer be blown away by explosives. However, environmental devices such as fusion coils are still affected by nearby ordnance explosions. Ranged weapon switching makes a return in Halo 5: Guardians, with the ability exclusively possible with plasma grenades. It awards the Combat Evolved medal when performed correctly.