I have seen some home made camo jobs that look pretty good. They base painted the rifle an OD green. Then attached various leaves from the woods they are hunting in, in a random pattern to the gun and sprayed on a tan color, then reattached more, different leaves, to it and sprayed on a light brown. It gives a very woodsy look. I do not think they used an air brush, just cans of spray paint. You could start with a light brown and then layer on OD green and then tan or any other colors that you think would blend in. Use Krylon Fusion, if it is on plastic.