To give you more options consider, getting a non pack gun carrier that you can get from eberlestock, and securely mating it(attaching it) to a different ruck.
Also, the Kifaru product Dogtown speaks of above, is called the GunBearer. I haven't used it before, but it would be good if you did not want constant wraparound protection for your rifle, or if you needed to get to it quickly. The downside is that if you are carrying a 10 pounds plus of rifle, not including the odds and ends associated with a precision rifle, and all the pressure from the weight will go to your waistbelt, and might end up annoying the hell out of you after a bit of rucking. It would work for something smaller though, like a carbine, or service length rifle.