To take a "shot in the dark"; to "take a stab at s.t."; to give s.t. a try.
The -un imp. focuses on what you are trying to get. The -an imp. focuses on the instrument used or the action one does in order to get s.t.
Take a stab at those rabbitfish (with your spear), hopefully you'll get s.t.
Take a chance and shoot, hopefully you'll hit something.