EBBIV - You can hit steel with a 12" .308 at 1000, or even 2000, if you have enough elevation in the turrets, and enough ammo. The issue with rounds like .308 or .223, that generally don't stay supersonic (faster than the speed of sound) out to 1000 or beyond, is that 1) many types of bullets can become unstable when the transition from supersonic to subsonic and 2) rounds start to drop A LOT as they slow down to those speeds, making distance calls very important, and wind calls infuriating. So all things being equal, if you want to avoid frustration and make hits at a specific distance, it can help to have a cartridge and barrel length that remains supersonic out to your desired range. To a point, the faster a bullet is, the flatter it shoots and the easier you can achieve hits at distance, but there is no free lunch and I love .308 because I don't like measuring my barrel life in minutes.
It isn't that you can't hit - for example, my 16" .308 is really consistent out to about 750 (I'm at sea level), sticking to around 2 MOA or so. But beyond that, my groups seem to open up to 4 or more MOA, and the wind calls become essential. It is fun, but I often feel like I am wasting ammo shooting further out than that. Whereas with 6.5 creed, for example, the wind calls, elevation, everything, is a lot easier at that distance. Not sure if that makes sense. If you said you only had access to a 500 or 600 range, most of us would likely say that either barrel length is just fine.
A 20" .308 is probably fine out to 830 or so, but if you have a particularly slow barrel (meaning when you chrono it, it tends to shoot slower than average), or try shooting 150 grain, or certain 168 grain loads, you might be on the transonic threshhold at that distance and just working a little harder than you would have to with a 24" barrel.
