I have three 7x57s , a model 700 remington, a 98 mauser built by mauser, and a bruno model 21 or 22 ( small ring mauser). I have not been able to get over about 2650 accurately with 139 gr hornady, or nosler ballistic tip 140 with any of them. and I have tried numerous powders such as IMR 4350 and H4350 4064 but none of the really modern ones such as big game hunter or RL17 etc. These loads were not really hot loads, but were full power loads, judging from extraction, look of primer etc. These loads were all extremely accurately averaging i inch or less for all power charges s I worked up. when they started opening up I backed off. but the chromo said 2600plus. I also have a winchester 70 7mm08 that I get 2850 from, accurately. I have sort of lost interest jn the 7x57 because I haven't been able to get adequate velocities from it.. Don't know what I did wrong. Its not as if Ill starve from lack harvested game as Im too much of a looney for that.
In a gun in good condition, 2,600 fps with a 139-140 cannot be referred to as full power loads, in my opinion. If you get that feeling from extraction or primer appearance it is because those signs are not a reliable indicator and, maybe, your are producing cases with a bit of headspace.
Your groups could be opening up and then tightening back once you pass through that sour spot. Or maybe not, as each rifle is an individual but what is a sure thing is that 2,600 fps is not a full power load with that bullet weight,
