Don't worry about it, it took me a while to figure it out too. But I only use standard ammo, so it's a useless feature for me. :laugh:
I only carry one type of ammo now for each gun I have. It seems the game wants to default to weaker bullets first. This way I have what I want.
In other words, the Ammo Swap feature is completely useless to me. I know other's use it, but let me move the dang thing so I can do something else with it. (I'm on PC so it is the 2 key)