Re: 2nd build but looking for some opinions
Firstly, I know this is going to be quite controversial, but I believe the .308 is rather over rated. For a medium range hunting round it can't really be faulted; packs a big punch and there's an abundance of cheap factory ammo or reloading components, but on 1000 yard targets it can be beaten hands down by a number of other rounds. Don't get me wrong, it will get the job done, but with more drop and wind drift than necessary. Although your requirement was just for it to remain supersonic at 1000 yards, which it will do.
As for barrel length, I would personally go with at least a 24". It may not be as manoeuvrable as a 20" but it will gain you about 150fps instantly. And when you can get free velocity like that you can't really complain. Also, with something as short as 20" there will be unburned powder coming out the end, which will give you a bigger muzzle flash and louder bang, could erode your crown, and is altogether just a bit of a waste of good powder.
Just my opinion, others may disagree!