UpHunter,
If you have already decided on the rifle and are worried about every ounce then why would you pick a long action. You have only one choice and it is the 308. The other calibers are 8 oz heavier. Not sure why you are asking your original question if weight is your deciding factor. Their is a lot of experienced hunters on here that are offering good advice.
If price is no object then build a custom rifle off of a pierce Ti action and get exactly what you want.
