It seems to me that if you're in a tank firing with for example HE rounds and one is in the chamber, and you decide you want to switch to AP there should be a reload time there. Instead you just hit the button and the round magically changes from HE to AP and is ready to go down range.
The other thing I find quite annoying about this when GVing is that if I'm firing HE or HVAP and pop up to the pintle to have a look around when I get back in the gunner position the ammo fairy magically switches my rounds back to AP.