Take the parts/s down to bare steel then apply some salt water to it either with a spray bottle, rag, sponge, etc and keep it moist for a few days. The rusty patina you're looking for will quickly begin to appear. The longer you do this, the more rusty the metal will get. I'm assuming you're working with steel and not aluminum or another non-ferrous metal such as brass or copper.
Once you've achieved the look you're after, rinse the metal with fresh water and let it dry. You can either apply a coat of flat clear or just leave it as is. There are products that can be applied such as 'browning' that will give you an old, antique look. Check your local gun shop for these and follow the instructions on the package. Good luck.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"