They were bored straight through. Originally the .38 Colt cartridge had an outside lubricated bullet. The bore diameter was .363 until the M1903 version came out and the diameter was reduced to .357 to better fit the inside lubricated ammunition. Link is to a PDF copy of the 1917 revision of the US Army manual for the revolver. Be patient.
https://ia902705.us.archive.org/2/items/descriptionofcol00unitrich/descriptionofcol00unitrich.pdf
https://ia902705.us.archive.org/2/items/descriptionofcol00unitrich/descriptionofcol00unitrich.pdf