I don't have the "beast" but I do shoot a 338AI and some 375CT's with the McMillian Supermag stock and it seem just about perfect for those rifles. They are extra long and have the sniper fill, have all the adjustable capacity for cheekweld & LOP too. Unless I was going with something much bigger I don't think I'd like a stock that huge.
I only place I've seen it was in photos or video from the Applied Ballistics team. I do know the in another 2 months or so McMillan should be releasing the gen 2 version of it so I'm holding out till then to order mine for a Cheytac imp. build.