This was the issue Norma Match ammo loaded with the 168 SMK, and with it's conservative muzzle velocity, and on a stinking hot, humid day was in the transonic zone before 800m. Didn't happen every day, but it happened more than enough to make life miserable for anyone shooting a C3 rifle.
I believe the general consensus is that the boat tail angle of the 168 SMK is too steep, and when it enters the transonic range makes is susceptible to stability issues. Groups may open up, or as it yaws and wobbles about it can loose velocity faster than it should and go subsonic sooner than it should.
Other clones of the 168 SMK would have the same problems.
The transonic range begins at about Mach 1.2 as the bullet speed drops below about 1340 fps, plus or minus.
That being said, I think the 168 Scenar is not a clone, and has a much shallower boat tail angle and should handle the transonic zone better than the 168 SMK. I suggest giving them a good try, and if they don't work, then I would gladly take them off your hands. I've seen factory Lapua ammo (167 Scenar) shoot crazy small groups out to 800m.
Otherwise, go lighter but faster, or heavier but slower. Lighter means less recoil, heavier means more recoil, so there is always this kind of trade off for you to decide on. Whatever works best for you and your style of shooting.
Couple of good reads here:
http://bulletin.accurateshooter.com/...-and-accuracy/
http://bulletin.accurateshooter.com/...bility-and-bc/
Take care,
Dave