Re: 20" 260?
The 260 has an advantage over the 308 in the wind right from the start, so you're already gaining. You have to figure an average 308 is using about 1 Mil more of wind over a 260 at 1000 yards. (10MPH) 3 MIls (308) to 2 MIls (260)
So using some software to check, as I don't have my 18.5" 260 data here, but I have some data from my other 260s, if you use 8.2 MIls to reach 1000 yards with a 260 and then move to a similar 260 that needs 8.5 Mils to 1000 yards the wind gain is .1 Mils over each other... so go .3 Mils more with a 8.8 Mil 260 you might need 2.2 Mils instead of 1.9 Mils, at least that is how it is tracking. Again, I don't have my data in front of me to say, but read the posts in the SH Build thread and I think I describe my wind hold.
In reality its not my job to convince anyone that shorter is better, but I will point out the fallacies that longer is so much better because the numbers just don't support that nor do the realities on the ground. I still recall when the guys first showed up to RO with 6.5 CM and they were crowing about it, and I laughed, which lead to the 1000 yard challenge... I hit on the first round with my 308, and beat the "wind cheater" in one shot, why simply because I know my round and knew what I needed in that wind. There is no substitute for understanding the rifle you are using.
But its 2010, we have better bullets, powders, and barrels... look at the 178gr Hornady Superformance load, it is puts the 308 back in range of the 260, but still it is the difference between 9.6 MIls verses 8.8 that is a 22" 308 barrel verses an 18.5" 260, still in terms of the 308, its huge gains for that bullet, but over the 260, just enough to be racy.