Your question is a bit too short and incomplete for me to give a specific answer, as you don't state whether rifle or revolver.
For Revolvers: Cylinder throats or exit holes should be fitted closely as possible, and for best performance MUST be slightly larger than the barrel groove diameter. The LBT lap kit explains how you can alter your gun if the throats are smaller than groove, and supplies materials and instructions to lap barrels on any gun.
For Rifles/autoloading handguns: As a general rule, the largest bullet diameter which will chamber will be the most accurate, even if several thousandts larger than the barrel groove diameter. The most certain way to fit rifles is for the customer to swage a throat slug (components availabe from LBT) and send it to me with the order. I'll cut a bullet that will fit and shoot accurately in that gun. If you have several rifles of the same caliber, send throat slugs for all and I'll make one bullet that will be short enough to chamber in all, and fat enough to shoot accurately in all, with sizing down possibly required for the smaller rifles.
