Cartridge Efficiency & Consistency
The goal of any
cartridge is to convert potential chemical energy of powder granules
into kinetic bullet energy. Therein, granules that do not ignite before
the bullet begins to move suffer four progressive deficits. First,
produced gases have less time to work on the bullet – which is moving
faster and getting closer to the muzzle. Second, as bullet velocity
increases, propellant push weakens – both molecular velocity and gas
flow velocity through a bore are limited. Third, compared to underlying
layers, energy production from deterred surface layers is lower. Fourth,
as bullet movement exceeds a few inches, confining pressure decreases
dramatically so that combustion occurs at a much slower rate.
Consider an example: 308 Winchester, 150-grain bullet, 50-grain charge. If a 10-grain plug of unignited powder follows the bullet into the bore, then initially, the effective bullet is 160 grains and the effective charge is 40 grains. Unless granules in this plug can ignite soon enough and burn fast enough to pressurize the developing void behind the accelerating bullet, that plug not only does not add to performance but its acceleration absorbs energy that could otherwise accelerate the bullet.
For these reasons, as the bullet progresses through the bore, energy conversion efficiency for newly ignited granules plummets. This reduction is dramatically non-linear. Compared to granules that ignite before the bullet begins to move: typically, granules igniting after the bullet has traveled one-fourth the distance to the muzzle contribute much, much less than one-half the projectile energy; granules escaping ignition until after the bullet moves halfway to the muzzle are unlikely to ignite and, even when ignition occurs, contribute essentially nothing.
As an example, consider a typical 308 Winchester load using VarGet and the 168-grain MK bullet. In that combination, once bullet movement begins, it takes only about 0.582 milliseconds for the bullet to move 5 inches (one-fourth total travel distance for that bullet in a 21.591-inch barrel, beginning with an overall cartridge length of 2.8-inches), see graph.
Thereafter, any newly unignited granule has only about 0.540 milliseconds to contribute to bullet energy (1.122 ms - 0.0582 ms = 0.540 ms), see graph.
Equally, the purpose of any accuracy-critical cartridge is to produce the most consistent possible ballistics. Everything in the following text is predicated upon understanding that ballistic uniformity is enhanced by early (consistent) granule ignition and, conversely, ballistic uniformity is degraded by late (typically inconsistent) granule ignition (and low loading density). Briefly, increasing delay between primer blast and individual granule ignition increases the resulting ballistic effect of minor variations in that delay – the ideal is achieved when all granules