At 600+ yards it will be just a tad better than the 340 wby and the same as the 338 ultramag and 338 lapua. Also it will be not nearly as good as stuff off the 378 wby and full length 416 rigby case. I have seen over a hundred 338-300 ultramags that some call an edge chronographed and never seen one aproach 3250 fps with a 250 grain bullet. Super fast rifles do exist way out of the norm but expect more like 3100-3150 fps tops. LTLR on here gives realistic unbiased average velocities all the time on big 338's. He would be your best scource since he was probably the first one to develop this cartridge and produce it in quantity back in the 90's and will give unbiased data since he doesn't have a dog in the fight.