In case you didn't understand Mako's pithy reply (pithy is good), he means the number 2 key. You can also select them directly by going to the ammo screen in your pipboy and selecting the ammo of your choice. Only ammo available for the weapon you have selected will show highlighted.
-gunny out.
You can also go into the game controls menu and manually choose any key or mouse button to assign as a 'swap ammo' key. Personally, I wish they had just done that instead of also robbing one of the (already too few) 1-8 keys.