I don't think Uberti has ever made a Walker with a .440 bore, unless you are referring to the land diameter. The groove diameter of the Walker is more likely to be about .454. The barrel likely has 7 lands and grooves, so if you slug it and mike it you will be measuring across one land and one groove opposite, so will not give you a direct measurement of either the groove or the land diameter.
As Curator said, the Walker conversion cylinders are chambered in 45 Colt.