strike the cartridge primer. The tip of the firing pin must
e. To avoid kinking, the outside diameter of the
be sufficiently long to indent the primer by the amount
spring should be made as large as possible. Since the
shown in table XI (0.025 to 0.030 inch). The rear face
outside diameter of the firing pin is 0.46 inch, it was
contains a 0.375-inch-diameter counterbore to
decided to use a 3/8-inch-outside-diameter spring.
accommodate the initiator spring, and a 0.250-inch-
Using arbitrary load-deflection tables, it is found that a
diameter long bore for the initiator pin. Three holes are
helical spring with an outside diameter of 0.375 inch will
spaced around the circumference of the firing pin for the
have a 0.0604 inch deflection per coil under a load of
locking balls which lock the firing pin and initiator pin
21.76 pounds when the spring wire diameter is 0.054
together.
inch. This spring force is between the required 20- and
35-pound pull, so this spring may be used in the initiator.
It has previously been determined that the spring should
compress approximately 5/8 inch. Ten active coils
should be used to provide this travel since the deflection
of each coil is 0.0604 inch. This yields a total deflection
of 0.604 inch. Assuming the ends of the spring will be
squared and ground, the total number of coils necessary
will be two more than the number of active coils, or 12
coils (total). The solid height of the spring will then be
Figure 82. Firing pin for initiator.
12 times the wire diameter of 0.648 inch. (It is
necessary to know the solid height because a spring
should never be compressed to its solid height.) The
with a groove on the small end and a flat section on the
free length of the spring may be found by adding the
large end. A hole through the flat section is used to
solid height to the total deflection (0.648 + 0.604 = 1.252
attach the triggering lanyard (or link) to the trigger. The
inches).
safety pin groove is located on the cylinder near its flat
f. The firing pin should strike the primer before the
section and is designed so that the initiator pin can be
spring reaches its free length, since the energy
rotated 360with the safety pin installed. The stopping
contained under the point of the curve diminishes as the
shoulder is located forward of the safety pin groove.
free length is reached. This also insures that a preload
The remaining portion of the initiator pin is smaller in
can be established.
diameter and is positioned within the coils of the initiator
spring. A groove is located immediately behind the
forward face. This groove is used to lock the firing pin
spring described above. It will be noted that the usable
to the initiator pin by means of steel balls.
deflection (distance from "cocked" to "strike primer") is
0.500 inch.
Figure 83. -Initiator pin (sear pin).
d. The initiator spring is designed to supply
sufficient energy to the firing pin to fire the M42 primer
in the head of the cartridge. The M42 primer has an all-
fire energy of 26 inch-ounces (table IX). The force
necessary to compress the spring (lanyard pull) must be
Figure 84. Spring force curve.
between 20 and 35 pounds. Since the lanyard travel is
specified as 3/4 inch (and "overtravel" must be provided
to insure separation of the firing pin from the initiator
pin), the spring should supply the required energy after
being compressed approximately 5/8 inch.
95
