Looking at the numbers, the .264 WM at 79 gr. H2O and the 6.5 PRC at 67 gr. H2O, the PRC is around 15% less case capacity than the .264.
But, with these new powders, remarkable performance is possible.
As aalf quoted George Gardner, "For the 135 a charge of 60.3 grains of RL26 yields 3200 fps. No pressure The 147 does 3000 with 58.2 grains H1000, light pressure . These are out of a 24" Bartlein 1-8 twist 5R. Loaded to 2.945".
That's strong. Lots of performance with less powder.
DF
It sounds like a ripper in a light’ish sorta rifle DF.
