You can only change ammo for the bastard gun (SMG's) and assault rifles. All you have to do is hold down the RB and you will Artyom change ammo. You will be able to see what ammo you're using because the military ammo glows a little. Just be careful though and not waste all your military ammo killing nosalis ;o) I only use the military ammo for the sniper rifle because it doesn't use dirty ammo. Or at least I don't think it does. The revolver and shotgun has only one ammo type.
Hope that helps you out.
Currently playing Metro 2033. Just bought Dragon Age : Origins : Awakening. Waiting for Just Cause 2, Splinter Cell Conviction