I had a Savage single shot varminter in .220 Swift. At the time I had access to several different kinds of premium factory ammunition, the best that they made. Surprisingly, the most accurate was the Hornady, loaded with their 40gr. Vmax bullets. At the maximum distance that you are planning for, I would at least give that bullet a try. I think that you will be pleased. Although the conventional wisdom is to favor heavier, higher BC bullets, I suggest that you run the numbers using some ballistic software before you make a decision. Having said that, I think that a 12" twist gives you more options.