"Magnum" has to do with chamber pressure. .357 Magnum is built to withstand a much greater chamber pressure than a .38 S&W Special, same for .44 Special and .44 Magnum. The velocity gains are from the greater pressure.

So when your load exceeds the chamber pressure of a .38 Special it's a magnum, more or less. I'm sorry that I don't have at hand right now what all those are.

I guess this might not be of much help and maybe not what you are looking for, but that's the definition.