The USGI drawing B7267079 for the operating rod spring is illustrative of the detailed
requirements typical of M14 parts production. Some of the drawing B7267079
specifications are described below.
The operating rod spring material was 17-7 precipitation hardening stainless steel wire
per military specification Mil-W-46078 and manufactured under military specification Mil-
S-13572 Type 1 Grade B. This material was chosen for the operating rod spring in 1952
as part of the T44 rifle modifications. The physical dimension and performance
requirements were as follows: wire diameter equal to 0.054 " + or – 0.001 ", coil outside
diameter of 0.4575 " + or – 0.0025 ", free length of 15.23 ", a total of 104 coils, and the
direction of the helix must be right-hand wound with closed ends.
The operating rod spring formed by coiling cold drawn wire had to be heat treated as
follows: age hardened at 900 degrees Fahrenheit for one hour then air cooled. After that,
the operating rod spring was heat set by heating at 700 degrees Fahrenheit for twenty
minutes with the spring compressed to a length between 5.9 " and 6.0 ". Heat setting a
spring improves the stress relaxation during use. Age hardening is used to increase the
strength and hardness of manufactured parts made from certain alloys, e.g., copperberyllium
and 17-7 cold drawn stainless steel. Each material responds differently to the
age (precipitation) hardening process based on a combination of time, temperature and
the amount of cold drawing from the initial rod size into the final wire diameter.
Depending on the material, the toughness, corrosion resistance, fatigue strength,
electrical conductivity or thermal conductivity can be improved upon by stopping the
hardening procedure before or after the time needed to achieve maximum strength. By
age hardening and heat setting the operating rod spring, service life was significantly
increased.
The operating rod spring, like other compression coil springs, was designed with a given
spring rate. The spring rate, or stiffness, is defined as the amount of load (force) needed
to compress the spring one inch. The lower the spring rate the softer the spring. After the
operating rod spring had been heat treated and heat set it was compressed to solid
length three times and then load tested at several specific lengths. These tests verified
the operating rod spring met the design spring rate. The design spring rate for the M14
operating rod spring was 1.95. When the operating rod spring was compressed to a
length of 10.97 " the applied load had to equal 8.13 pounds + or – 0.81 pounds. When
the operating rod spring was compressed to 6.42 " the required load was 17.00 pounds +
or – 1.69 pounds.
When the operating rod spring was compressed to the minimum operating length it
measured about 7.19 " long with an applied load less than 16.00 pounds but sufficient to
engage the bolt lock. The USGI drawing B7267079 for the operating rod spring specified
a maximum length of 5.78 " for the solid compressed length. The design and
manufacturing process ensured the USGI M14 operating rod spring would function
properly for thousands of cycles. It also means that the minimum force necessary would
always be exerted on the operating rod spring guide, and consequently, the magazine full
of life sustaining ammunition. The M14 rifle enthusiast or collector should proceed with
caution when hearing the term “mil-spec” in casual conversation with such detailed
design, manufacturing and testing requirements for USGI M14 parts.
