"the 9mm projo dia (.3555) is .0245+/- smaller than the 38 chamber dia (.380)"

That's interesting. I just measured the cylinder face diameters in one of my 9mm 637-2 J-frame conversions. They ranged from 0.356 to 0.360, which is identical to my .38Sp and .357Mag cylinders for the same gun.

I wonder why yours are larger?

My three 637-2 conversions are coke can accurate at a hundred feet, which is good enough to meet my needs.