When holding down Q, a set of icons for the weapon highlighted will show below the weapon grid. It has to be a weapon that can take alternate ammo types though. Highlighting each icon shows text of what ammo type it is.
Do note however that each time you change ammo type, your weapon reverts back to empty and you have to wait for the the entire mag reload process to cycle through, which can take a while with some weapons. So you want to be careful when and where you make such changes.