Re: NF 15 or 22 power for an AR10 SASS ?
There is no hard and fast rule that says that a particular caliber rifle has to have a certain power scope on it. Only you can say what you need. If all you are concerned about is hitting a man sized target, you'd be surprised at what you can do with a 10X. If you want to do some precision shooting, you may need the magnification. If you already have 22Xs, I'd say go for another.
Really, the bottom end on the 15X is 3.5X. The bottom end on the 22X is 5.5X. If you spend most of your time shooting at 100 yards or under, it might matter. If you spend most of your time shooting at beyond 100 or 150 yards, it doesn't matter much. If your concern is ranging and you think that 22X is too much, ranging on 11X isn't too much harder. I'd go for the extra power, if precision is what you want.