Re: .308 barrel length
I've owned and shot several .308's, as that cartridge is my favorite little pet. I've found unless you need 22" or more as a handling issue (mostly in hunting rifles), anything over 21" isn't beneficial. Some would argue a barrel that short will hurt you out to 1000 yards. I've used Remington M700 LTR's with 20" barrels and a few custom sticks with 20" and 21" barrels and haven't ever had a problem getting a bullet out that far. No problems so long as the nut behind the trigger does his part, anyhow.
Accuracy-wise, my best groups have come from 20" barrels. I don't shoot BR, though, so my experience is tactical and hunting rifles only.