The Standard Zero option of the GMDR.Load External Ballistic Calculator generates for the user the typical trajectory described by a specific bullet that has been zeroed at the "Zero Distance" specified by the user, and under strictly Standard Atmospheric conditions. These trajectories are those commonly tabulated in Reloading Manuals, and which most of the handloader community are very familiar with. The Standard Zero calculation is very fast since not major requirements are impossed on the firing conditions.
The angle of the barrel with respect to the scope (or line of sight) is obtained by the archaic Siacci's hypothesis of flat trajectory. The calculation then proceeds with the use of such angle estimate, but solving the actual equations of motion for the bullet flight. It is therefore a wonderful tool to investigate the limitations of Siacci's method for predicting trajectories.
When the "Std Zero" option is shown (by cycling through the three zero options for the zero-option button), and a bullet or factory load has been selected from the drop down catalogs, the projectile trajectory for the specified "Zero Distance" is drawn on the screen. If the ambient conditions are those of the Standard Atmosphere ( 0 ft altitude, no wind, 59 F of temperature, 78% relative humidity and 29.53 in of Hg of ambient pressure), the trajectory will show crossing the line of sight at the zero distance specified by the user.
To estimate the effects that changes in the enviromental conditions would have on the trajectory obtained at standard conditions, the user can dial different values in the appropriate boxes under "Enviroment" and see the changes in the newly calculated trajectory.
The storage of the different trajectories on the screen (by clicking on the "Add as Reference" button), allows for simultaneous access to the trajectory information. The presence of winds, shooting at altitude, variations in ambient temperature, etc, will generate trajectory changes that the user can easily access from the screen. The information on the most current trajectory can be displayed in tabular form. The specifics of all traces plotted on the screen, can be displayed by the user, by mouse left button clicking at any position on the trace.
For a fast and good general estimate of flat fire trajectories, and for the users that feel comfortable with the standard industry ballistic tables, the Standard Zero option is a simple and excellent tool. For the users that seek highly precise calculations, the "Local and Special Zero" options of the External Ballistic Calculator are the correct answer.