Re: 26" vs 16-20" .308
The following is from last year's Rifles Only school in December, when density altitudes were as low as -2000 feet. The details may not be perfect, but SH user jackinfl can verify the info -- it pertains to his son's rifle.
Most of us at the course had bolt action rifles with at least 23" barrels and we were shooting factory 175 SMK (Federal, Black Hills, Corbon) just fine out to 1,000 yards. Jackinfl and his son were the only ones using factory Hornady 168 (either AMax or TAP).
Jackinfl's barrel was in the 24/25-ish inch length and his son's was in the 22-ish inch length. His son's gun shot Hornady 168 quite well out to 900 yards on 1 MOA and 2 MOA steel. The results were horrible at 1,000 yards -- virtually nothing impacted 2 MOA steel, with impacts easily landing 2 MOA off target in random directions. His sub-MOA gun became a 4 MOA gun at 1,000 yards with Hornady 168 when the bullets went subsonic at roughly 900 yards.
Jack's son switched to factory 175 SMK loads (either BH or Corbon) the next day and was regularly ringing 2 MOA steel at 1,000 yards. We suspected the 175 SMK bullets were also subsonic at 1,000 yards, but at least they weren't yawing wildly.
BTW, jackinfl's longer barrel gun did just fine with Hornady 168 out to 1,000 yards. We figured the extra few inches of barrel kept his bullets supersonic to at least 1,000 yards.
A short barrel can get .308 bullets to 1,000 yards, but at what speed. Furthermore, that bullet's flight characteristics through transonic velocities will determine if respectable accuracy is even a possibility.
As others have stated, you must first understand your realistic target distances, then apply your rifle and ammo to best meet those goals.