Technically speaking, the bore should be .257" or therabouts. The groove diameter is the large measurement that can be from .263" to .268" or whatever depending on manufacturer.

I know it's nitpicking, & this is not meant as a put-down, merely as a nomenclature correction. We should use the proper names for the parts of our firearms or we will lose them over time. When I first saw the title in the list of threads, I instantly thought .257", but found the thread wasn't about the bore size at all.

Best wishes