I just got a model 12 VLP in 223 with a 1 in 7" twist but also want another barrel and I know I have to get another bolt face for it. I really want a larger caliber but just don't know what yet.
.308 is rarely a bad choice. They are accurate and easy to load for. However, they do have a bit of recoil in the heavier bullet weights that have the best BCs. The 6.5-.284 is a marvelous long range cartridge that fires high BC bullet at high velocity with relatively light recoil. The negative for the 6.5-.284 is that barrel life is short. Another choice would be a .260 Ackley Improved - almost the velocity of a 6.5-.284 with less powder and longer barrel life. A couple of the guys are getting a reamer for this caliber to try it. Another way to approach long range shooting is a fast twist 1:7.5" 6mm like a 6mm XC, 6x47 Lapua, or 6mm Remington (.243 has a reputation for a short barrel life) using one of the new 115gr bullets. Some 1000 yard Bench Rest competitors use a 6-.284. I don't know about its barrel life, but I am sure it is short. The 105 to 115 gr 6mm bullets are accurate, have high BC and low recoil. Not a bad choice either. I shoot a 6mm XC with 107 Sierras at 2900fps. I really works well out to 1000.
One reason the twist rates are how they are is probably because of the mass production of barrels they don't have to reset the equipment for custom twist rates.
Why not look at 7mm-08? The 7mm bullet is Superior to the .308 as far as ballistics, and most of the components are more readily available. You can get the same weight bullets with much better bc's.. Just food for thought. I am converting my .308 to 7mm-08 in about 4000 more rounds.