  A comparative study of simulated air shower longitudinal profiles is
presented. An appropriate thinning level for the calculations is first
determined empirically. High statistics results are then provided, over a wide
energy range, (10^14.0 to 10^20.5 eV), for proton & iron primaries, using four
combinations of the MOCCA & CORSIKA program frameworks, and the SIBYLL & QGSJET
high energy hadronic interaction models. These results are compared to existing
experimental data. The way in which the first interaction controls Xmax is
investigated, as is the distribution of Xmax.
