Parameters could be:
0:Object - unit that is reloading
1:String - Weapon that is being reloaded.
2:String - Muzzle of the weapon being reloaded.
3:String - Magazine class being removed from the weapon
4:String - Magazine class being reloaded into the weapon
5:Number - Ammo in magazine being removed from the weapon
6:Number - Ammo in magazine being reloaded into the weapon
To set the ammo in the old magazine to 10 on reload.
player addEventHandler ["Reload",{(_this select 0) setAmmo [(_this select 1), 10];}];