[Jack 1957]

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 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.
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.