If we figure 175gr FGMM out of an 18 inch barrel:
MV: 2460
Transonic @ ~875 yrds
Elevation correction @ 1000yrd: 43.5 MOA
Same load from 24 inch barrel:
MV: 2600 fps
Transonic @ ~950 yrds
Elevation correction @ 1000yrd: 38.5 MOA
According to Exbal, figuring speed of sound at sea level of ~1,125fps (speed of sound is slower at higher altitudes/lower barometric pressure). G1 drag model, nominal atmospherics. A whopping 75 yards of difference for 6" of barrel.
Mind you all that data is very dependent. Ballistic programs without a trued algorithm only really give you a guestimate, even with shooting through a chrony (even high dollar, recently calibrated chronies have about +/-20fps).
Tight bore .308s (.299) get much higher velocities. So do 5Rs. Shorter barrels are also known to have more consistent harmonics, as do heavier and higher quality barrels.
Focus on 600 and work your way out. With a .308, you will eventually hit that transition range no matter what. My round/rifle combo crosses it just fine @~850yrds (A guesstimate from a trued algorithm on Exbal).
If you want a .308 to go transonic predictably, the only solid advice I can give is do not use 168s. They are renowned for destabilizing due to a niche problem between bullet length and mass, and have a lower BC.
More mass typically means a higher BC, anything with a higher BC will make the transition smoother, among many benefits. Go with 175 or heavier, or some of the newer 155s (more expensive, but comparable). In the interest of fairness, the 175 SMK does max out at .505 above 2800fps, so if you could hit that magic mark, there could be some minor gains to wind bucking ability. In order to attain that, you need a +26" tube. The only other thing a few FPS of MV gives you is a slightly smaller elevation correction. Big whoop.
This is all assuming that this weapon is intended to be somewhat practical and not a bench shooter or F-class. If it is one of the latter, by all means, go with a 30" barrel, it won't hurt and it might help. If you are REALLY fixated on 1000, and not worried about commonality or price, there are better cartridges to get there on.
What I am really saying is: stop worrying about velocity. It is way way way way way way down on the list of things that can help you smack steel at longer ranges.
