What is the difference between the 38 Auto and the 38 Special other than one is only used in revolvers and the other is for Semi Auto Pistols.

The 38 Auto is shorter and has less case capacity as well as less velocity and energy. The 38 Auto also typically fires lighter slugs than the 38 Special.

So one is a longer round for a revolver, more powder and heavier slug. The other is a shorter compact round with less powder and lighter slugs for personal defense.