Have you tried equipping another gun?
If that doesn't work, try holstering it and drawing it a few times. (Double-tap triangle on PS.)
If that also doesn't work, try depositing it into the stash.
Sometimes, inventory elements get caught in a RAM buffer. Last thing I'd try is holstering, save in a new save slot, then exit the game and relaunch it. Reload that save and see if it will let you unequip or replace it.