Using The 203 attatched to the Weapon

[Ranger_Vet68]

Question.....For the "Under Slung" weapon, (203 or what ever you have on your weapon) I chose the Number 5 key binding for it. But everytime I hit the "Five Button" it never switches to that weapon......Is there something I'm doing uncorrectly to gain accsess to said weapon........Any Help & Suggestions would be a GREAT help to me.......

Thank You.