The question may need to be restated
tg,
I have a suggestion for a revision of your question.
How much pressure can a M686 stand?
I think I understand your thought process but.....having more room in a cartridge is no reason to pack it full of powder. As ArchAngel has stated, there are pressure limits per cartridge.
Now, according to SAAMI specifications, the voluntary pressure limit that a 357Mag chamber should be able to withstand is 140% of maximum pressure. In CUP it's 1.4 * 45,000 = 63,000CUP They rounded up to 65,000CUP in their manual.
PSI is determined the same way, 35,000 * 1.4 = 49,000 rounded up to 50,500psi.
Suffice it to say that you don't want to push those limits and certainly not from a 38spl case. The 38spl revolver doesn't have to be built to withstand those pressures according to those same standards. Don't push it.
I can almost tell you what will happen if you load a 50,500psi load into some 38spl revolvers. A 357Mag may take several, I think the standard requires 10 rounds, loads of that magnitude, some of the junk 38spl revolvers around won't take half of that!
Be safe.