(I'd just like to whine that the 60 sec. spam-prevention-delay-timer is a bit too long for my typing speed, haha)
It is not possible to change it in the .ini, and i think it is as you say....it's hard coded.
Did you already try it and it didn't work? Like I said, I never tried...I don't really mind the key. A little annoying but I became used to it.
*searches*
http://www.thenexusforums.com/index.php?/topic/250502-change-ammo-changer-hotkey/ (it's like the 7th post down)