Representation
of events inside a typical SMc cartridge immediately after primer
ignition.
Primer shockwave propagates though propellant column
while compressing charge toward front of case.
As primer shockwave
reflects from case shoulder, it focuses into propellant
mass behind bullet base, thereby adiabatically heating the entrained
air around that last-ignited and last-to-burn propellant.
Case shape
and shoulder design work to trap the greatest feasible percentage
of unburned propellant within case, thereby reducing apparent
recoil and barrel heating.
Ratio of case body diameter to bullet
diameter is also critical; see associated text for further explanation
as to why this design produces optimal performance.
In order to
create a more powerful SMc case, body length is increased
and shoulder is appropriately modified.