I don't know what gas size ports the 20 gauge barrel has, but here is what I would do. First, I would find the largest bit that will slip into the gas port without forcing it. Then I would take the next larger bit and push it pretty hard with my fingers while twisting it in an attempt to get it into the gas ports. Do this a couple of times and if there is any caked carbon on or in the ports, this should remove it. BTW, using a little mineral spirits to soak the ports beforehand might be a good idea.