the one that i really like is the 6.5-06 A-Square. excellent ballistics and is one of the few legitimate wildcat cartridges left. doesn't and hasn't been offered as a factory chambering. no one makes factory loaded ammo for it. they make reamers for chambering, and barrel makers will chamber a barrel for it. you can get dies for reloading and even one company, Quality Cartridge makes headstamped cases for it. some of the load data books show data for it. it's not rare, but having one pretty much gaurantees that not everyone will have one.