Okay, then how about 140 grain bullet @ 2550 on an80 degree day with 50% humidity and a BP of 30.00 and an altitude of 75'?
Your 140 Hybrid (G7 BC 0.317) is calculated to be doing 1,802 fps at 600; 1,373 fps at 1,000. (Berger Bullets ballistics data and free Point Mass Ballistics Solver 2.0 program.) That's around 1.2 MACH at 1,000, adequate but not startling. Perhaps more to the point is firstly wind conditions and the resulting drift if you miss a change, and secondly what competition you face in whatever discipline you'll use the rifle in.
10 mph @ 90-deg wind is calculated at 3.75-MOA at 600; 7.2-MOA at 1,000.
If you're up against somebody shooting a .284W, .284 Shehane or 7mm short magnum with a Berger 180gn VLD at 2,950 fps (not excessive for some of this cartridge list in F Class) his or her bullet performance runs as follows:
2,179 fps / 2.8-MOA drift at 600
1,734 fps / 5.2-MOA drift at 1,000
You'll know what sort of wind changes you might get on the range(s) you shoot over, the 10 mph / 90-deg just being a useful convention to allow comparisons, and a 7 or 5-MOA wind change between shots would certainly be exceptional on the ranges I shoot over, a complete miss by a large margin if not taken as the frames I shoot on are around 2.5-MOA from target centre to the edge.
As for your original point re getting the accuracy and velocity relationship optimised, that is a big issue for F/TR shooters like myself who use .308 Win at 1,000yd and occasionally beyond. The answer is - being simplistic / unrealistic at times - we want it all, small groups, high MVs and small muzzle velocity spreads. Generally, we do get it these days and there are F/TR shooters whose external ballistics today are not that far short of what a good 6.5 did in the early days of F-Class. That is the 155.5gn Berger at getting on for 3,100 fps; 185gn bullets at 2,800 fps + and 210gn or heavier bullets at 2,600 fps +, sometimes nudging 2,700 fps. The price is barrel life of course, since you get nothing for nothing.
My philosophy for 1,000yd has always been to go for the best grouping combination I can get at an acceptable MV and spread. That may mean losing out on another 100 fps, but it's a poor trade off if the faster load doesn't group or throws every 5th shot out as a flier. If the load won't group well, your developing match plot will almost certainly become misleading as a shot off to one side losing you a point may be due to wind, or maybe not if the group is running at an MOA or more, likewise poor elevation control makes it very difficult to know if you've got your sight correctly adjusted and whether you should be changing it or not.
On the other hand, a rifle / load that groups into a quarter-MOA or better may be severely constrained, even useless, if the terminal velocities are around the speed of sound and/or the opposition shoots rings round you because they just stay within the 10-ring and able to 'shoot through the wind', while the same conditions lose you a point every time you miss a minor change. In our GB F Class League rounds (ie the UK) we judge how hard the conditions really were after the event by the gap between the top F Class and F/TR scores. If they're fairly benign, the gap is close and the top F/TR shooter would have a top 3-5 place in 'Open' (even would have won an event outright in one of last year's rounds!); if there is a big gap, you know it was really was hard and you weren't just making excuses!
FWIW, 2,550 fps seems a bit on the low side for a .260 Rem with 30gn barrel. How is it throated - a long throat can make a large difference to the MV and in QuickLOAD if not input (via COAL). Likewise have you measured your case capacity using the water overflow method and input it into QL using actual fired case capacity? The program's default capacities are sometimes well out and if smaller than you're actually running will give an over-high pressure estimate.
It's all a lot less critical ballistically at 600yd, but the extra wind drift can still lose you points to competitors whose ammunition groups then same as yours, but is going that bit quicker / moving that bit less in the wind.
