Those Colt 38 Gold Cups were quite unique and somewhat rare. Back in the 1980's one of the shooters in our indoor bullseye league had one. If I remember correctly, they were blow back, and had serrations in the chamber for retarding the extraction. As nice as they are, they could not equal the S&W 52 in overall reliability and accuracy. It's a shame they stopped making them though.
As for measuring 5 groove bore diameters, it is quite difficult, but possible. I had several people measure my S&W 52 slugged bullet, with different micrometers, and came to the consensus, that the diameter was .355. That's the assumption I am going on.