If I was you, I wouldn't use the weapons in the game almost at all. I had much more success with just avoiding enemies entirely, or using my bare hands, than I did trying to hold them at gunpoint. The enemies in the game respond much more realistically then they did in the first one, and even if you could lower your weapon in front of a guard, I would expect him to attack you for it in this one.

And the two shoulder buttons are pressure sensitive. Press them all the way down to open your menus, and just press them slightly to equip or unequip whatever weapon or item you're using.