Compression springs are a critical component of firearm performance and durability. Making a recoil compression spring perform properly in the extremely limited space available in most firearms, and ensuring it is durable enough to sustain repeated use, frequently requires springs to be made of wire that is stronger than typical round spring wire. The options available include shaped wire or stranded wire.
Designing with shaped wire is facilitated by computer design guidelines and formulas for spring rate and equivalent direct (tensile or compressive) stresses. There are really no accurate programs for completely predicting load and stress on stranded wire, so designing with stranded wire typically requires an in-depth understanding of mathematical relationships, as well as more development and prototyping.
Space constraints make spring design tricky Spring designers use shaped wire for recoil compression springs in firearm applications because of space constraints when the spring is compressed. There is simply no room to compress rounded wire springs in the space allotted. By contrast, the shaped wire’s “solid height” (length of the spring when under sufficient load to bring all coils into contact with the adjacent coils) is much less than that of the round wire spring, giving it the required increase of travel distance.
Shaped wire is defined as wire with a cross-sectional shape other than round, and is usually produced by cold rolling. Typical shapes include square, rectangular, or “keystone”, a triangular wedge-shaped wire. Using chrome silicon shaped wire rather than traditional carbon steel material produces a spring that can withstand additional intense shock and higher heat.
Excellent computer programs exist for designing loads and stress on these shaped wires, developed by such trade organizations as the Spring Manufacturing Institute and the Institute of Spring Technology Ltd.
Stranded wire springs are especially suited to repetitive impact loading conditions. They are frequently used when impact stresses and velocities are so high that a single wire spring would not provide a long life cycle. Stranded wire consists of three to seven strands of wire that are machine-twisted or woven around each other, or around one wire that serves as a core, to form a single strand.
The spring designer typically purchases the stranded wire from a manufacturer, after specifying the number of strands, wire pitch, lay, and other parameters that match the design requirements. The stranded wire can be made of traditional music wire, which is manufactured out of tempered high-carbon steel, also known as spring steel, or from high tensile rocket wire, which can withstand even more compression. Stranded wire compression springs can damp migratory waves that traverse the spring under shock loading. If one strand breaks, the spring will still be functional and will continue working under stress even with some fracturing. Stranded wire springs are recommended when fatigue is the primary concern.
Figure 1: Springs made from different shaped wire and stranded wire
Military firearms have the most stringent requirements, and design specifications usually call for each component (including the spring) to be able to withstand 20-30,000 rounds of firing before it fractures and fails. The wire needed to achieve this performance standard in the compression spring may be significantly more expensive (almost double), but the life and death consequences of using the weapon in battle mean designers can easily justify spending a few cents more for a spring. This will avoid overstressing the spring or getting right to the spring’s performance limits, and may provide as much as 15 percent more performance from the weapon.
By contrast, commercial manufacturers may be seeking only about 10-15,000 rounds and may opt for a spring that will not wear as long, but will be more cost effective. Typically, military/government weapons are designed around the springs and their requirements, while commercial entities frequently try to get springs to work later in the design stage after sourcing other longer lead time components.
For an interesting look at how an "amateur" inventor did a lot of out-of-the-box thinking (plastic frame, for example) which led to the now standard Glock pistol, see the just-published “Glock: The Rise of America’s Gun,” by Paul M. Barrett. The first 1/3 is about the design and design process, the rest is about contracts, legal hassles, and other issues.