You need a Magnum bolt face for the .300 WSM, and a Standard for the .308. You also need a long action for the .300 WSM, but it can use the .308 as well. Switching barrels back and forth is not the best idea, if you want percision. If it's the smallest bit loose, or if the barrel is not perfectly screwed back on, you're looking as some shitty shots. Also, this would cause some HEAVY wear to the threading on the barrels, and would result in a loss of accuracy fairly quickly.