Wow... you just can't make this stuff up...
You do realize that if you're seeing pressure signs, ie flat primers, ejector marks, etc, that your generally in the 70,000 PSI+ range right? Blown primers to find pressure?! You've got to be kidding me...
Also, everything else being equal, velocity increase with case capacity is only about 1/4 the percentage of increase in volume. A .260 holds about 2% more powder than the Creedmoor so is only good for about 14fps more than the Creedmoor when loaded to equal pressures.
You may be getting 250 FPS more out of your .260, but if you are, it's because you are running significantly higher pressure than you are in your Creedmoor.
John