Could easily be evented.
You'll need 2 items for this. 1 the ammo, 2 the ammo box. Have a parallel process and have it check the Shift button for example for the reload. Check to see if ammo is zero, then check to see if you have 1 or more ammo box. Then have it take one ammo box away and add 6 ammo for example. Something along those lines.