Im a little confused as to why you would want to put a shoulder in at all...both 357mag and max are straight walled cases..... my understanding of a "snake load" is a shot payload fired through a handgun or rifle chambered in a straightwalled cartridge like these...so if you want to use a 357max in a 357 revolver...trim cases so they are NO MORE than maximum length of cylinder and load your shot INSIDE the case...maybe good thick wad of cardboard under shot a thin cardboard disc on top,cemented in place with couple of dobs of nail varnish etc to glue the disc to mouth of case and have at it.....the "payload" being front disc,shot and rear wad disc will all exit case and enter barrel ,encounter rifling and shove its way through ,hopefully not generating too much spin in process.....where is problem??? this is in effect how a steel sided .410 is loaded except a wad is used to hold payload...