allows the almost instantaneous radiant ignition of all exposed granule surfaces within the initially unignited mass so that the unignited plug is much shorter – despite the fact that it is composed of harder to ignite granules that burn more slowly!
Quantitative Support
Similarly, QuickLOAD,
a software program that models internal ballistics, based upon how
individual granules burn, predicts very accurate results for typical
bottlenecked cartridges, such as the 308 Winchester; this fact suggests
that it models internal ballistics reasonably accurately when the
vast majority (perhaps >90%) of granules are ignited before the
bullet has moved more than about two inches – which, as in the bottlenecked
20mm cannon, is the situation for the 308 Winchester. However, in
bottleneck cartridges with a wider and steeper shoulder, the program
tends to under predict velocity, perhaps due to delay associated with
shearing a particularly long plug. Predictions tend to run about 100
fps slower than actual velocity.
The 458 Winchester Magnum has a propellant chamber essentially identical to the 7mm Rem Mag – same diameter, same taper, same length. The only significant differences are that the 458 case is not bottlenecked and typical 458 loads use a much faster (less deterred and hence more readily ignited propellant). In the 458, and in all similar cylindrical or near cylindrical cases (where no shoulder exists that is sufficient to disrupt the moving plug of unignited propellant), QuickLOAD predictions are always dramatically and systematically faster than actual performance. Predictions run about 150 fps faster than actual velocity.