The caliber of the firearm determines what ammo is most appropriate. As of 2015, Browning offers 39 different rifles in the X-Bolt series, with calibers from 7mm-08Rem to 204 Ruger.
Caliber refers to the diameter of the bullet and, indirectly, the diameter of the rifle's barrel chamber. Browning X-Bolt rifles come in 20 different calibers. If the caliber is unknown, then a chamber casting must be done. This entails removing the barrel and pouring a substance down it that hardens quickly. Cerrosafe is a clean, safe casting medium made of bismuth, a coarse crystalline metal that melts between 158 and 195 degrees Fahrenheit.