(b) The firing pin tip is another
(3) Firing Pin Guides.
The firing
important consideration in firing pin
mechanisms of propellant actuated
design. A hemispherical nose tip is
devices are designed so that the
used to transfer the kinetic energy
end of the firing pin guide contacts
of the firing pin in a concentrated
the cartridge head, a condition
pattern and thus secure good primer
referred to as "zero head space"
indent.
Such a tip, however,
requires accurate alignment of the
supports the head of the cartridge
firing pin, guide, cartridge, etc., or
against "primer blowback", but also
excessive off-center strikes will
determines the firing pin protrusion.
occur. Reliable operation demands
The tip of the firing pin must indent
that the firing pin not strike more
the primer sufficiently to fire the
than 0.020 inch off center of the
primer, but it is equally important
primer cup.
that the firing pin does not pierce
the primer or gas may escape
(2) Firing Plugs.
through the pierced primer. Firing
(a) Artillery-type primers may be used.
pin protrusion and the diameter of
in cartridges using firing plugs
the firing pin tip (to avoid pierced
seated in the cartridge head over
primers) depend upon the primer
the primer cup.
used. Table XI presents the desired
This plug (fig. 33) allows a greater
protrusions and diameters for the
amount of off-center striking by the
four primers currently used in
firing pin.
When the firing pin
propellant actuated devices.
strikes any portion of the plug, the
Table XI. Firing Pin Protrusions and Diameters
plug strikes the primer with a
Primer
Firing pin
Firing pin tip
minimum of eccentricity since it is
protrusion (in.)
diameter (in.)
guided.
The system suffers,
M29A1 ..................... 0.025+0.005
0.075
however, from reduced sensitivity,
M42.......................... 0.025+0.005
0.040
since the firing plug does not
72M.......................... 0.030+0.007
0.075
transfer all of the energy from the
50M.......................... 0.058+0.010
0.093
firing pin to the primer.
(b) At present, firing plugs are not used
b. Gas-Operated Firing Mechanisms.
as much to compensate for off-
center strikes as to prevent "primer
(1) The firing mechanism should be designed
blowback", by backing up the primer
so that the firing pin develops sufficient
with a firing plug and its guide. This
kinetic energy to fire the cartridge reliably.
makes the firing mechanism more
This reliability is achieved in gas operated
critical, since it is often difficult to
designs by the proper choice of shear pin,
provide a sufficient amount of
firing pin weight, firing pin cross-sectional
energy.
The
"firing
plug"
area, and firing pin travel.
arrangement is used to prevent the
escape of the high-pressure gases,
developed in the chamber during
locked-shut firings, around the
primer cup ("primer blowback").
Figure 34. Gas-operated firing head.
Figure 33. Firing plug.
39
