I have been fooling with both the 6.5 GAP and 6.5 PRC this year quite a bit. One of the more interesting results involved measuring the water capacity of each case with the same 143-grain Hornady ELD-M bullet seated to the "standard" overall cartridge length. In the GAP 4s this was 2.85 inches (to fit in the standard short-action magazine), and in the 6.5 PRC 2.95 inches, the maximum SAAMI overall length.
The H20 capacity in both, using fired Hornady cases, was within a couple 10ths of a grain. While the GAP case body is wider, it's shorter than the PRC's body. According to the laws of interior ballistics, what one will do, the other will as well.
Now, if bullets are seated out another .1 inch in the GAP, it gains 1.3% more case capacity. Since any difference in capacity results in about 1/4 as much difference in potential velocity, given the same pressures and barrel length, this gives the 6.5 GAP about a 10 fps advantage in loads getting around 3000 fps.
