Define the difference of a short and long action rifle?

Answer 1

After WW2 a development occurred in the development of gunpowder. After that development a "full power" cartridge could be made smaller -- shorter -- than before.

Short action rifles are simply shorter versions of their longer brethren... shorter and infinitesimally quicker.

As an example, the external ballistics of the .308 Winchester/7.62 NATO are identical to the external ballistics of the .30-06 Government. The smaller .308 cartridge gets the same performance by using the newer "triple-base" or "ball" powder.

The advancement in powder technology allowed experimenters to develop a lot of shorter cartridges.

The shorter cartridges probably did not feed as reliably in long action rifles. In many actions the cartridge has a moment of freedom after it is stripped off the magazine and before it enters the chamber. Using a short cartridge in a long action is likely to reduce the reliability because that moment of freedom is longer.

As the shorter cartridges became popular, weapon manufacturers started offering rifles with shorter receivers to accommodate them.

An M1 Garand is a example of a long action. An M14 is an example of a short action.

Weapons chambered for .223 and the AK47 cartridge may not "qualify" as short action rifles, because those cartridges are not full power.