The Particle Velocity calculates the velocity using the speed of sound, peak on-side pressure and ambient pressure. This is part of the Rankine-Hugoniot equations.
Note: speed of sound in dry air (343.2 m/s) as the default. Condition outside of dry air (e.g. underwater) will warrant another value.
Resource:
"Formulae for Ammunition Management." International Ammunition Technical Guideline. 1st ed. New York: UN, 2013. 2-3.UN SaferGuard. United Nations Office for Disarmament Affairs (UNODA), 20 May 2013. Web. 12 June 2015. <http://www.un.org/disarmament/convarms/Ammunition>.