.38 Spl shells can be shot in .357 mag but they are still .38 Spls. Pressure is determined by the volume of space that is available for your powder to burn in. The initial high pressure spike occurs before the bullet leaves the case. Too hot a load in a smaller .38 Spl case could create a pressure spike greater than the strength that a .357 mag cylinder can hold.
.38 Spl shells make great low recoil shooting from .357 mag revolvers. If you want the power of a .357 mag, load .357 mag cases.