I want to check the bore diameter of my Omega.
I've got digital calipers.

I haven't seen any posts on this.

Take a ball of the same caliber and flatten enough to be larger than the bottom of the rifling use a flat ended short starter to push it in the barrel take out the breach plug and carefully push it back out on to a soft cloth and measure it. This is not the best way but it is good enough for all normal purposes.