Cable locks are useless. It takes less than a second with a pair of heavy diagonal cutters to get a cable lock off. I've had to do it when I lost the key to one. After I realized how easy it is do destroy them, I haven't bothered to put one on any of my guns since.
Why not just pull the bolt and carrier and lock those up seperately? Much smaller and easier to secure, and the rifle isn't much of a rifle without it.