But leave silly people like me wandering around wondering why I can't find a .38 revolver.
The .38 special cartridge is ".38" because of the nominal diameter of the *case*. It is, in fact, the same diameter as the .357 magnum (which has a .357 caliber *bullet*). The difference between the two is length and allowable maximum pressure. The .38 special was designed in the days of black powder, and has a quite low maximum pressure, while the .357 magnum is designed to operate at higher pressure and thus launches the same bullet at significantly higher speed. .38 special is safe to shoot in firearms chambered for .357 magnum, but not the reverse: this is why the .357's case is longer, to prevent it from chambered in firearms that it could potentially cause to fail.
It's actually a little strange that there are no .38 revolvers laying around in F:NV, because they are extremely popular and common. It would be interesting to know which problem would be a larger obstacle to their inclusion: The additional artwork needed, or the additional programming needed (because the .38 revolver can't use .357 magnum ammo, but the magnum can use .38 special). But then there's also no R51 assault rifles, which is an even more glaring omission since we know that these were mass produced as the principle US assault rifle at the time of the Great War. Oh well.
As a further bit of trivia, there also exists a .44 special, which shares the same relationship with the .44 magnum.