If the penetrants don't work, try EVAPORUST. You can find it lots of places. Put your gun in a plastic tub and cover with Evaporust for a couple days. It eats the rust without harming the metal. It's not an acid like vinegar, so none of the drawbacks of that. Take it out, rinse with hot water, and oil immediately to prevent the part rusting again. And save the solution, it's reusable. Check it out on YouTube, it's fantastic for cleaning metal. My son did 2 of my Dad's wood planes, they came out like new, and never hurt the paint.