125gr bullets at full power 1,450 fps are known to crack the forcing cones of K frame revolvers in the thin area. Since your model 66 is a K frame and S&W no longer makes or stocks the barrels for K frame .357 mag revolvers you run the risk of ruining your gun with no way to fix it short of finding a used undamaged barrel if you choose to shoot the light 125gr full power loads. A better option is to use the Remington Golden Saber at 1,250 fps which is less likely to damage your barrel.

Here is an article on the issue of the damage 125gr loads can do the any K frame .357 mag: http://www.gunblast.com/Butch_MagnumLoads.htm