I have had good results with 260 Lapua. Of course, the chamber will determine whether you will want heavier/thicker brass on the neck. I know my McGowen 260AI really likes the 260 Lapua brass. Very tight chamber.

I also have a Shaw 260 AI that Apache Gun Works just made for me. I am using Nosler brass for that one (I wanted a different brand for each of the two AI's) and I notice the neck on the Noslers open up a little more than the McGowen. I did not think to measure the outside diameter of the fired brass and compare them before reloading them, so I cannot say for sure if the difference is the brass or the chamber dimensions. Good articles from Accurateshooter.com indicate that when Lapua started making 260 brass, it made it pointless to try to form them from anything else.

Boots: I agree. I find fire forming the brass with actual bullets is not all that wasteful. Its surprising how well they shoot while doing so.