Reloading manual data is obtained in several ways:
Up to about a generation ago, all the loading manuals developed their data based on test guns using copper crusher and tarage tables. The method used was firing test loads in a standardchamber and barrel that blew out the case wall to operate a piston to compress a copper slug. The amount of slug deformation was read on a "tarage table" chart that translated to copper units of pressure and compared against chamber pressures of ponds per square inch. SAAMMI (sporting arms and ammunition manufacturers institute) chamber and barrel specifications, and pressures were used to establish maximum loads. Before this, some loading data was developed based on reading primer signs and measuring case head expansion.
With advancements in electronic instrumentation (namely piezoelectric transducers and storage oscilloscopes) more accurate psi measurements were made, and most all loading data found today is based on actual chamber pressure measuring methods rather than the indirect tarage table methods. The ammo factories and all powder manufacturers use this method.
The only real problem with under mininum loads in the .357 Magnum is if you use the slow burning powders like WW 296 or H110. These are ball propellents, and undercharging them by more than 10% can give very erratic ignition or even the "KAH-BOOM" secondary explosion effect where the powder detonates instead of burns, and can wreck your gun.
WW and the loading manuals warn you of this, and advise against reduced charges of these and similar ball powders.
HTH
John
