Build the .300 WM. You already have the barrel. I'd favor the .300 for elk. Although a well placed shot with the 6.5 will definately put an elk down. Then again heavy well made 6.5 bullets are legendary for penetration. Hey, you what!!?? Convince yourself that you need the 6.5 and send me the .308. Just kidding. Build the .300 WM or better yet build a .300 RUM (I love mine) You will need to do a little work on the feed rails (probably for the PRC too). And then when you can swing build a PRC? After all, a man can't have too many guns.