I have looked at a bunch of sources and can't find the case length for the .357 Atomic. But it WAS longer than the standard .357 and Herters made a point that while the .357 Magnum could be fired in their guns, the Atomic could not be fired in a standard .357.
The .357 Atomic, .357 Herters, .357 Herters Atomic, .357 Great Western Atomic, were all the same thing. For a while it looked like the round might take off, but several problems, including the gas cutting that plagued the .357 Maximum, put a stop to it. I have been told the case length was intermediate between the .357 Magnum and the .357 Maximum, but have also heard it was the same length as the .357 Maximum. It is not listed in my edition of COTW or in any other cartridge book I can find, even though there were a number of guns made and marked for it.
I think some people may be confused because as the ammunition went from scarce to non-existent, most gun owners switched to the standard .357 Magnum so others believed it was the same cartridge. But I don't think it was.