Would trigger whenever given unit switches weapon, as in switches to secondary, handgun, etc..
Should probably also trigger when a new weapon replaces a weapon in primary, launcher or handgun slot (by picking up a new one or via scripted changes)

<unit> addEventHandler ["WeaponSwitched", {}]

Would return:

• unit
• new weapon
• old weapon (if any)
• new weapon magazine data (like Reloaded eh)
• old weapon magazine data (like Reloaded eh)