Having difficulty visualizing what you are saying. Are you saying the case mouth flares out or crimps in as the bullets are seated?
A flaring/recovery curve just behind the sizing point. An extruding phenomena.
Excerpt from published in Precision Shooting, 2009, James Boatright: Yielding of Brass Case Walls in the Chamber:
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When compared with actual measurements, calculated bullet seating forces seem to be running too large by a factor of about
two to three. I can identify two of the possible geometric, mechanical effects that are ignored in this simple “radial expansion” analysis,
but that would cause us to calculate significantly smaller bullet seating forces if they could be included.
The largest effect being ignored could be termed the “transition cone” effect.
As the bullet is being seated into the case neck, it pushes a traveling “pseudocone” of neck wall material ahead of its base.
The cone angle in the vicinity of the leading edge of the base of the bullet would increase disproportionately as larger
amounts of neck expansion are attempted.
After being swaged up, the brass material in this area tends to “overshoot” the actual bullet diameter because it needs to re-bend
into a cylindrical shape once again. The tensile stress stored in the case neck is reduced by this effect throughout the “expanded cylinder”
portion of the enlarged neck when compared to the simple radial expansion calculated.
In fact, this reduction in neck tension and in bullet seating force builds up so significantly with increasing neck expansions that it
becomes dominant and accounts for the relative maximum in neck tension that we find at about 1.5 to 2.0 mils of neck expansion.
The other significant effect being ignored can be called the “Chinese finger-trap” effect.
This “friction multiplier” effect comes about whenever we attempt to slide a thin-walled sleeve upon a mandrel having a
slight interference fit inside that sleeve. If we push the sleeve from the rear (as we are doing here in bullet seating),
both the normal force of the sleeve upon the mandrel and its resulting internal friction force are significantly reduced. [An axially
compressed, short, thin-walled cylinder will tend to assume the familiar “barrel shape” as it enlarges in diameter with each of its two ends constrained from freely expanding.]
However, if we were to pull the sleeve along by its front edge (as would happen during“bullet pulling”), the friction force on the mandrel
inside the sleeve would be greatly increased by the converse of this same effect. Hence, we have the familiar “bamboo,” or
“Chinese,” “finger-trap” mechanism that we utilize in woven wire to enhance the “grip” of cable-end pulling devices.
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This is an intelligent person considering
calculated -vs-
observed seating forces and stress/strain (tension).
It's been done many times over past decades,, always demonstrating IMO that there is more to it than can be consolidated to simple.
So, it seems ludicrous that folks simply coin interference fit, and seating friction, as 'tension', while they are clearly three separate things that do not directly correspond.
Bottom line: We need a tool to directly measure neck tension, and to
know what our bullet release preconditions actually are.
