After several years of trying to keep an S-A mount stable on an M1A; I concluded that the problem wasn't the mount, it was the rifle.
My M1A is history, and I now use a 20" AR-10-ish platform. Using IMI 168 Semi-Auto Match, I have no hassles. I also have a 308/168 FGMM-ish handload.
I carried the M-14 for over a year in combat and simply loved the thing. After getting the M1A I came to understand that the scope was (at least for me) somewhat of a compromise and mostly a losing proposition. Others make it work but for my part a good design shouldn't require so much engineering simply to keep it stable.
Greg