[Roy Allain]

Is this a custom rifle? Or a factory rifle?

I a factory rifle, you need do nothing. Any cartridge will fit.

If a custom, do the following:

If you can, load one (1) empty unprimed brass with the bullet of your choice. Then measure the neck OD with a micrometer. The demension is what your basic cartridge will be when you insert it into the chamber.

Again, if a custom chamber, what is the chamber neck demension? Subtract .003 from that measurement. That is what your loaded round MUST measure to chamber properly and also release after firing.

There is much more, but we'll take one step at a time.

Roy