All smokeless powders burn rather slowly at normal pressures (that is, one atmosphere, or 14.7 psi). These powders burn considerably faster when pressures go up into the thousands of psi. These powders have a performance "envelope" at which the burn behaves in a predictable, stable and consistent manner. If you try to run a powder at lower than its optimal pressure, you can get odd behaviors, like detonation, pressure spikes, bullets that stick partway down the barrel while the burn continues and pressure builds up again (effectively against a barrel obstruction). Not everyone believes in detonation, but there is no argument that light charges of slow powders, especially in voluminous cases is an iffy proposition, best avoided.

Fast powders are more forgiving of lighter-than-minimum loads. The main danger there is if you load so light that you have a bullet that does not leave the barrel. That sets you up for an obstructed barrel and all the disaster that goes with that.

But think about this: If you take a .357 Magnum cartridge and set the bullet deep enough that the overall length is 38 special length, you effectively have a 38 special in your chamber and can use the same load data a 38 special uses.

Or, you could increase the 38 special load data proportional to the increase of the interior volume of the case (under the bullet) and get similar performance as if you loaded 38 special brass. I do this all the time. From http://www.thehighroad.org/archive/index.php/t-768888.html