.36 cal (which is of course closer to .38" while modern .38 cal. is closer to .36" GO FIGURE??)
A lot of confusion exists in the naming of calibers, but this one is really no more so than a lot of others. The calibers of ML'ing rifles, pistol, & cap-n-ball revolvers were all named according to their bore sizes. .36 cal revolvers thus had bores in their bbls of .36". The barrels were rifled with groove dias of about .375" so the chambers were bored for this size ball, the .36 cal cap-n-ball actually firing a .375/.376" ball. The first cartridge guns were actually conversions of these & a heel bullet was laoded with both case & ball having a diameter of about .375 but for some cause they elected to name it according to bullet dia & simply rounded to two places & called it a .38 cal. Eventually the bullet size as well as the bores were dropped to use a .357" bullet which would put the grease grooves inside the case, but the .38 designation was not changed until introduction of the .357 Magnum. all makes "Perfect" sense.
