I had an experience very similar to yours when trying to achieve some high-velocity performances with 120 grain bullets in a couple of Ruger 7X57mm rifles. One was a M77, the other a No. 1A. I was using WW 760 powder, with which I was able to achieve 3000 FPS with 140-grain bullets, so I assumed 3100 would be possible with the 120 grain. WRONG! The best I was able to achieve with acceptable pressures was around 3020! In fact, I passed the " crossover point" , where pressure was continuing to rise, but velocity actually dropped a little!