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.
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.
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.
Depending upon what they are trying to achieve with the spring design and the envelope they are working with, those designing compression springs for the firearms industry sometimes try to "fit 10 pounds into an 8 pound box" – by designing a spring that simply cannot handle the load and stress and give a reasonable life. Many spring designs are overstressed, approaching the limit of the spring design capability when designers want to do more than is theoretically possible with the space assigned for the spring. In an area that is three inches high and a half-inch wide, they may be designing a spring that requires five inches of space. With an overstressed design, the spring will take a permanent set, losing its length and load.
To guard against this, spring manufacturers frequently undertake significant development and prototyping to assist the customer accomplish the right spring design.
For example, one major firearms manufacturer began its design process for a 40 millimeter (mm) pistol recoil spring using standard music wire. Standard music wire often just does not have high enough tensile strength to support load requirements and testing indicated the spring could not take the shock. During the spring consultation and prototyping phases of the design process, Connecticut Spring & Stamping CSS, which has a 70-year history and diverse expertise in developing stranded wire and shaped wire springs, recommended the use of chrome silicon flat wire. The chrome silicon material, widely used in the manufacturing of pistons in the automobile industry, can withstand higher heat and shock than music wire. The firearms manufacturer adopted the recommendation and the spring made from the chrome silicon flat wire was successful.
Another example was developed for a prominent 45 caliber pistol OEM, which started out using stranded music wire for its recoil spring. Testing showed that the material would not stand up and keep the load. While it wasn't fracturing, the wire's tensile strength was just a bit too low. A material change to rocket wire enabled the spring to achieve a higher load.
As discussed, a formula for designing stranded wire compression springs is extremely complicated; mathematical calculations are not available in a computerized program. It is therefore extremely important that the spring designer understand the complicated mathematical relationships required to work with stranded wire. Experience with a variety of different wire sizes and varying load requirements shortens the development cycle. CSS has been manufacturing stranded wire springs for many different OEM's since WWII.
It is also important to work with the wire supplier to clearly define specifications for stranding, especially the specific pitch the wire should be woven to. The spring designer should specify the precise wire output needed before coiling the spring.
CSS typically prototypes the spring by purchasing five to 10 pieces of each stranded wire being considered, testing perhaps three designs. Designers may make prototype variations out of both music wire and rocket wire, with different numbers of coils in the spring (say, 8, 10, and 12-coil versions), and in different wire sizes. First article load testing is conducted, and inner dimension, outer dimension, and the solid height of the fully compressed spring are measured.
These prototypes are then sent to the customer to be tested in the firearm and the preferred option is selected. Sometimes, the gun designer may even take one of these compression spring prototypes, cut out one of the coils and send it back to be duplicated for the final version. The last step in the process is adjusting the final print to meet the agreed upon spring design.
The choice of stranded versus shaped wire, and selection of music wire, high tensile rocket wire, or chrome silicon depends upon the spring design and what the engineer is hoping to achieve. Careful selection of wire base material, wire manufacturing specification to achieve accuracy, as well as knowledge of design formulae for spring rate and equivalent direct (tensile or compressive) stresses, and finally, prototyping expertise, will ensure a compression spring that works well.