Shotshell Reloading Background:
Technical Fundamentals
by Johan Loubser
The fundamental difference
between a shotshell cartridge and a typical center fire rifle cartridge
is, that the efficiency of the shotshell cartridge is 100% dependent on
the round itself. By this we mean that all the "resistive forces"
must be generated within the confines of the round itself. No assistance
is provided by the gun.
The reason for this is that the Maximum Peak Pressure is reached long before the base of the shot/wad assembly has left the case. In the case of a CF rifle cartridge, the peak pressure is achieved when the bullet is engraved, therefore the large contribution, as a result of lead/free-bore dimensions on the combustion process. (i.e. bullet/bore interface fit, bullet hardness, bearing surface, etc.).
The reason for this is that the critical engraving force which is so important to the dynamic combustion process present in a CF rifle caliber is totally absent in a shotgun. Shotgun and typical straight-case handgun calibers are actually basically the same in their fundamental ratios and dynamics. A shotgun can be described as an oversize low-pressure handgun caliber. That's why the same powders are used in shotgun and handgun calibers.
This means that the efficiency regarding ignition (and the subsequent increase in pressure) is totally controlled by the integral configuration and assembly of the round itself. These constitute the main inertial mass (shot mass), the initial internal volume (wad design), the dynamic collapse (primary expansion) of the internal volume (collapsible section of the wad), plus the displacing of the internal assembly and the unfolding of the fold/crimp (secondary/Final expansion).
The
way this COMBINATION interacts, will determine the efficiency Pressure
impulse (Profile and time-base) and the Peak-pressure vs Velocity
ratio (P/V). The resistive force, presented by friction in a shotgun is
negligible.
Crimping:
(Assembly)
This is certainly one of the most important aspects of the Shotshell reloading
process.
The influence of crimp on the ballistics is often ignored, and assumed
to be of lesser importance than primers and wad make/design.
The fact is that the effect of Crimp-strength can totally overshadow the
influence of the other components and parameters. This is controlled by
the following:
o Crimp depth: Depth setting on crimping machine.
o Condition of the case: Material hardness resilience.
o Wad: Length, Stiffness and rigidity/flexibility of collapsible
section.
o Wad Tension: Pre-tension/compression setting on crimping machine.
Some reloader's want to extend case life and they tend to crimp as shallow
as possible, and with the least crimp strength possible. However, this
practice can be problematic if the improper combination of primer, case,
wad and powder is used. Example: If a "soft" combination is
used, it can lead to underperformance, or in extreme cases, bloopers.
It is always wise to use a strong a crimp as possible, for any particular
load/combination.
Primers:
It is well known that different primers deliver different energy levels.
The way each company manufacturers and formulates the chemical composition,
and configures the hardware (metallic) parts of the primer, all
plays a major role in how the primer will deliver the energy to the powder.
Again, we must emphasize that it's all about the particular combination,
and whether a change in primer will show a difference in ballistics.
Shotgun primers are very sensitive to firing pin energy. This is due to
the proportionally large displacement/deformation that must take place
when the primer's cup is crushed.
It is extremely difficult to pin the data down to a standard one load,
where primer X will always deliver higher performances than Primer Z.
AAC developed our loads using a standard typical primer. If all conditions
are the same, the difference between primers will rarely be dangerous.
It is obvious that if the load one is using, are already running at the
maximum level with primer X, it would be unwise to merely change the primer
and continue loading. This is also true for any change in component or
procedure.
Once any component or procedure is changed, the combination will react
differently. The proper way to proceed is to reduce the load by about
0.5grain to 0.7grains, and then confirm the performance by measuring the
velocity or sensing the recoil/flight time. The reloader can then adjust
back to the same velocity/recoil level, by increasing or decreasing the
charge mass, or by adjusting to a stiffer softer wad or decreasing or
increasing the crimp.
Volumetric
loading/dispensing:
Always confirm the "thrown weight' from any bushing, bar etc on a
scale.
There are just too many variables that can influence volumetric measuring,
of powder. Some of these are:
The physical action with which the loading machine is operated.
The atmospheric conditions especially Relative Humidity (RH). This is
especially true of single base powders which are very hygroscopic. AAC
powders S1000 and S1250 are single base powders; N100 is a double base
powder.
My
friend, Johan Loubser, is considered by his peers to be one of top two
or three finest senior ballisticians in the world today. Johan currently
toils away at Western Powder Company, home of the Accurate Arms and Ramshot
lines of powder in Miles City, Montana. I'm delighted and thankful that
he has consented to have some of his technical observations published
right here.
Copyright 2007 by Randy Wakeman. All Rights Reserved.
