most reloading manuals give a velocity difference of +/- 23fps per in for a .308
now every rifle is different, different chambers, bore, groove, twist, contour, etc… all factors that come into play.
best way to answer this is just search for what muzzle velocities people are posting they have for those lengths, plug that into a ballistic calculator and see the results for yourself. if its paper punching it doesn't matter much, if it is for hunting you might want more velocity.
that being said my 16" semi auto does just fine on steel out to 1,000 but I would not hunt with it past 500 out of personal preference.
i have a feeling within 50 comments this will turn to blondes vs brunettes...