The only thing I can tell you is what works for me and why. It is ONLY my opinion and what works for me. Others will dissagree and will express their opinion as well. But from those opinons, you may be able to peice together the answer you want. Also, like S1 stated, it depends on what you want. If you are trying to set a new world record, I am in the same boat and am still learning what that is going to take. But if you just want to punch paper at 1000 yards for kix and competition, what works for me is the standard 308 winchester with Sierra Match King bullets. I use 155s, 168s, and 175s. I dont generally use the 155s unless I have a need for speed. Out of my barrel they only group from .250-.500. They will almost always group at 7.5" at 1000 yards. The 168 works for 100-600 yard benchrest shooting and Those can be shot out of my gun anywhere from .090-.150 Then comes my favorite, which is the 175. It groups about .250 most of the time and this round fired from my barrel yeilds MOA for MOA at 1000 yards, in other words whatever the MOA at 100 it is the same at 1000. As far as barrel length for the 308, I believe that 24" min and 26" max is perfect. Why? because less and you dont get the right burn rates for long range shooting, and longer and you get too much barrel whip and that hurts accuracy. I choose a 24" becuse it is a little more ridgid than the 26" and I still get good ballistics at all ranges. I also have a twist of 1-11.25" which is optimum for the 175 30 cal, 1-12 is best for the 168, and a 1-13 to 1-14 will suit the 155 best depending on the velocity you choose to launch them. If you want to figure that out, a formula given here can be used, this is the greenhill formula:
T=150(d/r) for velocities from about 1500 to 2800 fps. Substitute 180 for the 150 value for velocities exceeding 2800 fps. "T" is the twist rate. "d" is the bullet diameter. "r" is the bullet length to diameter ratio (length of bullet divided by diameter). In .308, it works out fairly well. Sierra 168 grain Matchking is 0.308 inches in diameter. Bullet length is about 1.210 inches so we have a length to diameter ratio of 3.929. Plugging this value into the formula and using the 150 constant (the proven best velocity for the 168 grain MK is 2550 - 2600 fps), we get T=150 x (.308/3.929); T=150 x 0.078; T=11.76 inches or 11 3/4 inch twist …. One rotation of the bullet for every 11.76 inches of barrel traveled. This is very, very close to the "standard" 12 twist barrels on alot of 26" Remington 700s. The 175 Matchkings do not perform as well in the 12 twist barrels. Again diameter is 0.308 inches. Bullet length is 1.257 inches, resulting in an ld ratio of 4.081. … plugging into formula (optimal velocity is 2500 to 2550 fps so we will use the 150 constant again) T = 150 x (.308/4.081); T = 150 x 0.0755; T = 11.320.
Barrel legnth is up to you, for max velocity 26" can be favorable, for max accuracy, shorter might be the wise choice. You will be suprised how little 40 or 50 FPS makes at 1000 yards vs. accuracy. As far as why I choose the 308, without going into lots of $$$ in experimenting with wild cats, I belive their are 2 standard cartridges that are perfectly ballanced for optimum accuracy, 1. the 308, and 2. the 50 BMG. They are both the prefered sniper weapon of the military and both very popular on the match scene.
I hope that helps a little.