I hope this will help others as it has helped me some. I know this is more of a camo topic, but since it is what I found for the m24, that it fits here.
I have the military version with the fluted barrel, which made it a little easier to do this, but what I found was that #31 o-rings fit just right on the outer barrel. I used some jute i dyed in about 16 inch lengths and looped them around the o-rings using the fluted ridges to help.
I am sure this might even be easy to do while off the rifle, but they are pretty tight going on. I hope this helps the next person in need of barrel camo.