ICAO vs Metro

The drag functions supplied with the Ballistic Explorer program originated from work done at the U.S. Army Aberdeen Proving Ground. The following dates were gleaned from Chapter 1 of Modern Exterior Ballistics by Robert L. McCoy

In chapter 8 McCoy states that US Army Ordnance used the Army Standard Metro (Metro) atmosphere until 1962, so as expected the standard drag functions, which were long in use by then were calibrated to standard Metro conditions. We have found no record of any of these drag functions being revised when the International Civil Aviation Organization (ICAO) atmosphere was adopted by Army Ordnance in 1962. By this date the military was using Doppler radar to create drag functions specific to each type of ammunition issued by the US military.

William C. Davis writes in the instructions to the ATRAG program that "The ARMY STANDARD METRO atmosphere was established at the US Army Aberdeen Proving Ground and was used for many years by the US Army as the atmosphere for which all standard firing tables were computed. This standard atmosphere was also adopted by the manufacturers of commercial ammunition, and it is still in use by the major manufacturers of commercial ammunition and bullets."

The G1, G5, G6, G7, GL, GS, and RA-4 drag functions included with Ballistic Explorer were, with permission, derived from the SAAMI Tables Of Siacci Functions And Drag Coefficients of June 1976. This document references SAAMI Exterior Ballistics Centerfire And Rimfire Ammunition of June 1976. This reference document specifically states in equation 19 that the standard atmospheric pressure to be used is 29.5275 in Hg for an air density of 0.0751265 lb/ft3, which is the Metro standard pressure and air density.

Because the long-used drag functions were defined and published for standard Metro conditions, Ballistic Explorer calculates ballistic coefficients referenced to standard Metro conditions. This is also true for the Oehler equipment Dexadine has developed software for, which includes the System 43, 83, 85 and 88. This reflects the current SAAMI practice and provides compatibility with legacy data.

Some manufacturers of commercial ammunition and bullets publish ballistic coefficients based on ICAO standard pressure and density. Multiplying a standard Metro based ballistic coefficient by 1.018 yields the corresponding ICAO based ballistic coefficient, which makes these published ballistic coefficients seem higher, but introduces an error when used with industry standard SAAMI drag functions.