I laid the bag right on the rifle and that gave me a good distinctive pattern, if you hold the bag away from the rifle, you might as well not even use the bag because it blends the pattern so much you cant really see it. Instead, keep the bag on the rifle and vary the distance you spray to do your pattern. The lighter and darker shades of the black showed up very well against the OD green and gave a very good looking (but functional) pattern.
I sprayed a base coat of Krylon OD paint over the entire rifle and let that dry. Afterwards, I used the Krylon flat black all over at varying distances. I had flat brown, but it didnt show up very well against the OD, so I didnt bother. The best thing to do is to practice on an extra stock or something before doing it on your rifle.