I have four weapons equipped at the moment: wrench, pistol, machine gun, and shotgun. Now I've killed the first Rosie Big Daddy, and it dropped its Rivet Gun. When I point at the Rivet Gun as in the screenshot its name is displayed suggesting it's a recognized object that might possibly be looted, and from what I've read that should be the case.
However, when I press E (or any other key) nothing seems to happen, as if I can't interact with this object at all.
What's going on? Can I actually loot this item now? Do I have too many (4) weapons equipped at the same time? Can I drop some to pick up this one?