Hey, this seems like a really stupid problem, so hopefully it has a ridiculously simply solution that I'm not finding. I've set my controls different than what the default is, so now my "next weapon" is 2 and my "previous weapon" is 1. But when I go into sniper mode and see that I have different ammo types I can't select them. For example, I see 3 or 4 green lights indicating that I have other ammo types, but I can only cycle through 2 types.
A specific example is: in the beginning when you first get the mortar and break the glass, you then get explosive sniper ammo. I can use this ammo type right away, but as soon as I cycle away from it I can never access it again. Any ideas why?
Thanks in advance guys!

Meanwhile, I realized the solution myself and have beaten the game.
To select different ammo for your sniper mode you choose the corresponding number (1-6). If you assign your items to the same hotkeys, then they will function the same way when not in sniper mode.