Ruger chose to make their screws with a concave slot in them, the theory behind this being that one could use a coin to remove or tighten the screw. In practice, this doesn't work very well for me, so I ground a screwdriver to fit.
Standard screwdrivers tend to slip, for a couple of reasons. First, the slot is shallow on the edges, so you can't get a real good bite on the screw. Second, the slot is relatively wide, and most of your screwdrivers won't fit it. And it's always important to have a good-fitting screwdriver any time you're working on a gun.
Simply loosen the screw and slide the barrel band forward off the stock and barrel. You may need to pry the bottom ends of the band apart with your screwdriver, so it doesn't bind on the stock and damage the wood finish.
Be careful - the barrel will now raise upward somewhat in the stock. But don't try to take it out of the stock just yet... go on to the next step.