First and most important - follow the manufacturers' instructions. In general, disassemble the gun according to instructions, and clean the mechanical parts (gas piston, ports, etc) with a good solvent like Hoppe's #9. Never use anything harder than a bronze brush. Similarly, scrub out the barrel from the breech end, starting with a bronze brush soaked in solvent, then patches placed over a jag of appropriate size until the patches come out clean. Inspect for any lead or plastic fouling that may need extra attention. Finally, dry the bore with clean patches and swab in a LIGHT coat of oil. If in doubt about anything, refer to manufacturers' instructions.