The AutoMag was originally designed to use a rimless version of the .44 Magnum cartridge; the .44 AMP. Shortly after it was introduced, though, the company Brough out a second cartridge, the .357 AMP. This was simply the regular cartridge necked down to use .357″ projectiles. It was a bottlenecked case with a huge powder capacity for a 9mm round, and it could be loaded to velocities in excess of 2000fps with a light bullet. Yowza! That said, a more typical loading was a 158gr bullet at 1500-1600 fps.
What made the second caliber feasible was the easy swapping of barrel assemblies on the AutoMag pistol. Since the case head was the same for both rounds, simply dropping on a new barrel was the only step in changing from one caliber to the other. Guns were even offered as one frame, two barrel combination packages form the factory.
AutoMag manufacture never became stable enough for the .357 AMP to mature as a cartridge. The ammunition was never offered by a commercial manufacturer, and it was always entirely in the realm of handloaders. Factory barrel options were a 6.5″ vent-rib and 8.5″ plain barrel; this is the longer pattern.