Gun swap through pickup?

I have had the same question, and thankfully, @AldoNaletto had the answer for it.

http://answers.unity3d.com/questions/294476/weapon-pickup-and-change.html

Use this code by @AldoNaletto, First you need to have 2 weapons, primary and secondary. Tag the primary one "MachineGun" and the secondary "RocketLauncher". After applying the code on your controller, put a check mark next to MachineGun. Now, put your secondary weapon on the floor as a gameobject and tag it "RocketLauncher". Create any collider and re-size it to be big. Check the "Is trigger". Then play your project, walk over your weapon on the floor, press "f", and the magic should happen hopefully. Don't forget to set your primary and secondary variables as your weapons in the inspector.

 var hasRocketLauncher = false; // tells if you have a rocket launcher
 var hasMachineGun = false; // tells if you have a machine gun
 var ammoClips = 1; // tells how many ammo clips you have
 var rockets = 10; // tells how many rockets you have
 var Primary : GameObject;
 var Secondary : GameObject;
 private var inTrigger = false;
 private var object: Transform;
 
 function OnTriggerEnter(other: Collider){
   inTrigger = true; // the player entered the trigger
   object = other.transform; // save the object transform
 }
 
 function OnTriggerExit(other: Collider){
   inTrigger = false; // the player left the trigger
 }
 
 function Update(){
   // if player inside trigger and F pressed:
 
   if (inTrigger && Input.GetKeyDown("f")){     
     switch(object.tag){
       case "MachineGun": 
         hasMachineGun = true; // enable switching to the machine gun
         Destroy(object.gameObject); // destroy the picked object
         SelWeapon(1); // select primary weapon automatically
         break;
       case "RocketLauncher":
         hasRocketLauncher = true; // enable switching to the rocket launcher
         Destroy(object.gameObject); // destroy the object
         SelWeapon(2); // select secondary weapon automatically
         break;
       case "AmmoClip":
         ammoClips++; // increment ammo clips
         Destroy(object.gameObject);
         break;
       case "Rocket":
         rockets++; // increment rockets
         Destroy(object.gameObject);
         break;
     }        
   }
   // Code to switch weapons when 1 or 2 is pressed:
   // - key 1 selects primary weapon:
   if (Input.GetKeyDown("1")) SelWeapon(1);
   // - key 2 selects secondary weapon:
   if (Input.GetKeyDown("2")) SelWeapon(2);
 }
 
 // function used to select weapon: activates only the selected weapon and 
 // deactivates all others; only activates a weapon if the player already has it
 
 function SelWeapon(nWeapon){
   Primary.SetActiveRecursively(nWeapon == 1 && hasMachineGun);
   Secondary.SetActiveRecursively(nWeapon == 2 && hasRocketLauncher);
 }