AmmoGuide has a Rechamber/Rebarrel calculator that isn't idiot-proof but will tell you if one chamber's SAAMI envelope (e.g. 6.5 Creedmore) is smaller at every point than another (e.g. 260 Remington). Join at AmmoGuide.com. It's a neat site for all reloaders. Pay attention to what sinman says - chambers can vary a good bit depending on the 'smith.