Usually a light weight silicone oil is your best bet. Don't use alcohol as it will destroy the coating inside the barrel. No reason to remove the barrel just back off the hop all the way. Cut strips of shop towel so there will be no lint, spray with a bit of oil and use a cleaning rod. Using a drop of silicone oil when inserting the co2 canister keeps the rubber seal in good condition and helps provide a better seal while at the same time makes it easier to remove the co2 if it's a tight fit or if you forget to take it out after playing.