In response to compression and fluidization, granules obtain a new shape and a greater packing density. Thereafter, very rapidly, those granules can ignite and propellant pressure will become sufficient to move the bullet. As noted, whether a granule ignites before or after the bullet begins to move is the critical point.

A Better Idea, Maybe
I was quite pleased when we applied the above noted considerations to the 6mm Thermos Bottle, with its essentially maximized body-to-shoulder radius. This design interacts in each of the above four areas as follows:

  1. It is trivial to prove that for a conventional cylindrical case body and for any given volume and diameter, this shoulder design provides minimum possible interior case area.

  2. It is reasonably simple to demonstrate that, so long as neck diameter is not significantly larger than or smaller than one-half of case body diameter, this design approximately maximizes both granule flow rate and convection at the shoulder-to-neck juncture.

  3. If initial primer blast penetration into the powder column is similar to what I have hypothesized (see sketch 3), this design also minimizes distance between effective flash front and remotest powder granules.

  4. This design does a splendid job of focusing primer shock wave energy where that is needed (see sketch 6), as explained in the following discussion.

A final consideration in favor of the Thermos Bottle shoulder design relates to primer blast shock impingement upon the case shoulder. Here, as opposed to what happens with a conventional shoulder, a shock wave striking near the perimeter is deflected very slightly and then reflects again and again until it reaches either the powder column directly behind the bullet or the boattail bullet base (where it strikes at a high angle so that it is reflected back into what will become the trapped powder mass behind the case shoulder, which is also beneficial). Similarly, looking at the next shoulder increment (the radial zone just closer to the neck), that parcel is initially reflected at a slightly greater angle and therefore ends up, on average, focusing

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