There's about 2 grains difference in case capacity, in favor of the .260. Since both cases hold around 50 grains of powder with a bullet seated, a 4% difference, this means there's about a 1% "advantage" in potential muzzle velocity for the .260 with the same bullet weights, on average, when using the same powders and bullets, loaded to the same pressures.
Obviously different rifles with vary slightly, but if you feel 1% is enough to go .260 (meaning around 27 fps with 140-grain bullets) then you should also consider the fact that long, high BC boattails can be seated near the lands in a 6.5 Creedmoor with a standard 2.84" magazine box, but not in a .260. Thus accuracy will usually be somewhat better in a Creedmoor with high-BC bullets, which makes up the 1% difference in potential velocity in less than 75 yards.
A longer magazine will solve this in a .260, but overall the Creedmoor has so many advantages these days that unless you want to use cheap .308 brass to make .260 cases, there's no reason to choose the .260.
I am sure many .260 fans will argue this, but I have experience with the .260 going back almost 20 years, and plenty with the Creedmoor as well.
I really like your reply, BTW.
The main reason why I criticize the 700/722 short actions is due to my perception of the inability to seat bullets due to short magazines with some cartridges. Making it difficult to get top accuracy with certain cartridges, especially with 257 Roberts, 244, and 6mm Remington. But even the 308 case based cartridges are limited, in my opinion. What was done with the Creedmoor, (close/similar to a 300 savage AI necked to the proper caliber) the ability of seating the bullet as needed is restored to some extent.
Building a 6.5 Creedmoor or a 260 (or even the x57 cartridges) on a Mauser action (or similar length actions) and the advantage of the Creedmoor design lessens, again in my opinion.
(I own a few 722 and short action 700's BTW)
In short, I think that in selecting the cartridge the action and magazine length should be taken into consideration. If I were to buy a factory short action 700 and felt that a 6.5 was needed and felt that the rifle was going to be used for other than woods hunting deer -- such as antelope or prairie hunting -- I'd go with the Creedmoor.
But then again, I prefer the longer actions.
