At the moment if you've got one weapon in your hand (eg a LAW Launcher) and your M16 on the sling on your back, if you drop the LAW (by going into the inventory screen, highlighting the LAW and pressing X), when you return to the in-game screen your M16 will still be on your back, with your hands empty.
To switch to your M16 you need to go into the quick-slot menu and equip it (two button presses).
Could I please suggest that when you drop the weapon from your hand, you either auto-equip the gun from your sling, or you can equip it by pressing Y (which should be the primary / secondary weapon swap button anyway.
Thanks!