Rapid and effective ignition of pyrotechnic countermeasure decoy flares is vitally important to the safety of expensive military platforms such as aircraft. QinetiQ is conducting experimental and theoretical research into pyrotechnic countermeasure decoy flares. A key part of this work is the development and application of improved ignition models to improve the understanding of the ignition processes occurring for these flares. Typically, these flares are wrapped in a material such as foil. Until this material bursts or ruptures, the flare can be considered to be effectively a closed system. Therefore it is possible to use gun internal ballistics models to investigate the ignition and combustion processes in these flares. Previous work validated the computer models for conditions of atmospheric pressure and 21ºC. This paper describes computer modelling and experiments conducted to validate predictions at pressures equivalent to those at 20000 and 40000 feet.