I beg to differ, 2" should be worth a minimum of 100fps in .308, perhaps even 150fps. The .308 is like any other cartridge and is not indifferent to barrel length- longer is better.
WRONG. .308 is not like other cartridges and longer is NOT always better.
FACT: a 24" lightweight barrel will NOT necessarily be more accurate, than a 16" medium or heavy barrel. There's more barrel "whip" (vibrations) in longer, thinner barrels. Therefore longer is NOT always better. BTW, take your 24" thin barrel and cut it down to 18 or 16", and you've just made it stronger and less prone to that "whip" and therefore MORE accurate.
Regarding velocity, there is on average only 20 fps (+/-) loss per 1" of barrel length with a .308. Average .308 Muzzle Velocities (MV) are between 2500 - 2900fps (standard loads). Therefore, as Chuck Hawks states:
"For rifles with muzzle velocities in the 2501-3000 fps range, the change in velocity for each 1" change in barrel length is 20 fps."
Magnum calibers or calibers with higher inherent velocities have a bit more loss in velocity per 1" of barrel loss.
"For rifles with muzzle velocities in the 3001-3500 fps range, the change in velocity for each 1" change in barrel length is 30 fps.
For rifles with muzzle velocities in the 3501-4000 fps range, the change in velocity for each 1" change in barrel length is 40 fps. "
"According to Gurney, who reports having routinely engaged steel targets as far out as 1,000 yards during testing, the .308 Win. cartridge is one of the least “sensitive” to reduced barrel length." (taken from the first link below)
http://www.americanrifleman.org/articles/ruger-gunsite-scout-review-2/
http://www.youtube.com/watch?v=RFhM0NHsYLk (check out around the 39 second mark)
http://www.tacticaloperations.com/SWATbarrel/
http://www.chuckhawks.com/rifle_barrel.htm
