IIRC Wayne Schwartz and/or Charlie Dell (SSRA and authors of The Modern Scheutzen Rifle) sectioned then available 38Spl, 357Mag and 357Max cases and found them the same; they are likely made from the same 'basic brass' stock just as 25-35, 30-30 and 38-55, etc. are.
Regardless, the chamber contains the pressure, the brass conveniently contains the primer, powder and projectile.