Hi:
What is the bore diameter differences between .38 S&W caliber and .38 Special ?

Bore diameters on both calibers varies from 0.348"-0.352".

Nominal .38 Special groove diameter is 0.357"-0.358". This can vary in production periods and manufacturer to as small as 0.354" in Colts.

Nominal .38 S&W groove diameter is 0.361"-0.362" in S&W Victory models. It can be even larger in the old-style break top models of non-S&W manufacture.

These groove dimensions is one reason why .38 Special bullets usually only shoot with so-so accuracy when loaded and fired in .38 S&W guns.