Action length is the distance between the front and rear action screws. These screws are found on the bottom of the stock fore and aft of the trigger and
magazine/loading port, which hold the barreled action to the stock.
When taking this measurement remember to measure from the middle of one hole to the middle of the other hole. Also, do not mistake the trigger guard screws for action screws. Since the difference between a short or long action can be as little as ½-inch either mistake can be critical.
Another means of identifying action type- is the cartridge type. In most cases cartridges measuring less than 2.8″ will utilize a short action. Long actions will not exceed 3.340″. Calibers larger than this will generally use a magnum action, or a special purpose action specifically designed for that cartridge.