3) Minimizing Distance between Effective Primer Blast
Ignition Zone & Remotest Powder Granules
Here By and I agreed that
we do not have sufficient information to settle a significant point
of contention. Does the primer produce a jet that tends to ignite
the powder column from the axis toward the perimeter, which By believes
might be an important mode; or, does the blast tend to force the charge
base forward while rapidly spreading to full case diameter, and thereafter
ignite the powder column from the base forward, which I believe is
the dominate mode. We agree that reality is somewhere in the middle.
Perhaps surprisingly, regardless of actual ignition mode, minimizing distance between primer blast and remotest granule in a cylindrical column requires the same shape – column diameter is twice column length. See sketches 2a & 2b.
What seems a more realistic representation of reality follows: As the primer blast penetrates into the powder column, proceeding from base toward shoulder, the leading edge rapidly extends to full case diameter, while developing a roughly hemispherical face. Refer to photograph of unconfined primer flash and sketch 3.
Regardless of specific details of actual ignition – which seem almost certain to vary, depending upon powder type, charge density, primer type, total case volume, flash hole configuration and several other factors – it seems evident that in this regard, a short, fat case provides a significant advantage.
4) Maximizing Initial Heating in Powder Column
Directly Behind Bullet
To understand the importance of this point,
we must first realize that any granules in the column directly behind
the bullet that do not ignite before the bullet begins to move will
thereafter simply follow the bullet into the bore as part of a single
clump. Thereafter, ignition of the forward portion of this clump is
likely to occur only at the base. This means a good percentage of
these granules will escape ignition for a relatively long time (and
until the bullet has proceeded far into the bore). Equally, we must
recognize that anything resulting in increased granule preheating
will reduce subsequent ignition delay.
Importance of this factor likely depends upon what percentage of the total charge is included in this zone. Here, the shorter, fatter case excels because, for an equal case volume and