The 357 Auto Mag is based on the 44 Auto Mag necked down to .357. The basis for this was the 308 Winchester case shortened to 1.298 inch. The 357 Auto Mag was announced in 1973, about three years after the launch of the original 44 Auto Mag but, as opposed to the 44 the 357 was never loaded commercially. (Erlmeier, Brandt Ref. 342A)
