I was going to ask exactly the same question, but for different reasons. We know that the 357 Mag case is a bit longer than the 38 Special case. Some people claim that the 357 case is stronger than a 38 Special case and can withstand pressures beyond what a 38 Special case can stand. I've seen people post on forums that the 38 Special cases can only withstand 20,000 CUP, just above the 38+P pressure. Other people have posted that 38 Special cases and 357 Magnum cases can each withstand 100,000 CUP, but I do not know who is right and who is just repeating Interweb fiction.
This discussion involves the results of possible loadings that exceed the published data and are inherently dangerous. This discussion is for educational purposes only and should not be the basis for actual practice on anyone's part.
I have thousands of once-fired 38 Special cases, and less than a thousand 357 Mag once-fired brass. I use a Winchester 357 Magnum Model 94 Trapper for shooting at the range. The 38's, even 38+P just pop out like a 22, and I want something more than that, but factory-loaded 357 is too much to enjoy shooting if I shoot more than 30 rounds or so. So, I want to load some of the 38 cases with a higher-than-38+P load, but not quite into the 357 territory, using Titegroup. I was going to work up from the highest load for 38's and go up by 0.2 grain increments to about 1 full grain above the published 38+P load. For the 125 grain JHP's that I am going to use, the max load for 38 is 4.6 grains of Titegroup, and the 38+P load is 5.0 grains. The lowest 357 load is 6.8, but it is for the 357 cases, which are larger than the 38 cases. Same load in a 38 case would be higher pressure. To work it up, I was going to go from 4.6 grain up to 6 grains in 0.2 grain increments, monitoring cases for pressure signs, chronograph results for velocity, and group size for accuracy. I am thinking that 6.0 grains in a 38 case is probably beyond the pressure generated by 6.8 grains in a 357 Mag case, but hopefully not beyond the pressure generated by max 357 Mag load in a 357 case.