By Dale Pereira & Pete Marut, Connecticut Spring & Stamping
Firearm manufacturing has come a long way from the days when a room full of gunsmiths was needed to ensure accurate, quality weapons. Nonetheless, manufacturing firearm parts still requires a deep understanding of how the many small parts work together to create a successful weapon.
With the rapid growth in the firearm market, together with trends that are moving toward smaller weapons and those made out of polymer, it is more important than ever that firearm part manufacturing make use of design development, prototyping expertise, and manufacturing innovations.
Firearm springs and stamped parts
Springs and stamped parts make up a large percentage of today’s firearms. There can be as many as 20 springs in a typical weapon, which may include compression springs used to resist applied compression forces or to store energy in the push mode, helical extension springs that store energy and exert a pulling force, and torsion springs with coils that are twisted rather than pulled to store energy. From as small as a compression spring with a .050 outside diameter to as large as a spring made from .062 wire or a recoil spring that is 2.5 feet long. The springs are essential to the proper operation of such components as the ejector, magazine, and recoil assemblies. Springs have to perform properly in the extremely limited space available in most firearms, and must be durable enough to sustain repeated use.
Firearms springs can be made with music wire, high tensile rocket wire or stranded wire, and can be made of many materials, depending upon the application. Chrome silicon, stainless steel, 17-7 PH and 17-4 PH are the most frequently used material because of their strength, durability, and corrosion resistance.
Stamped firearms parts include trigger bars and assemblies, slide stops with springs, and safety assemblies. Similar to springs, the stamped parts also have to operate within a very constrained space. Trigger bars must fit within the envelope of the gun and not interfere with the spring. The link from the trigger to the sear (the part of the trigger mechanism that holds the hammer back until the correct amount of pressure has been applied to the trigger) has an extremely narrow profile to operate properly without getting in the way of the trigger bar components.
Process starts with development and prototyping
The key to manufacturing quality firearm springs and stamped parts is engineering and support services, including making recommendations to improve manufacturability throughout the design and development process.
A good design process depends on good communication. At Connecticut Spring & Stamping, after an initial phone or other contact, the customer sends a drawing or CAD and experts offer feedback about the design and the need for secondary operations or machining. There may be several meetings or conference calls, when everyone can look at the screen and make necessary changes. Working through all these changes may take numerous online meetings or face-to-face visits. Prototyping is then used to assist the customer accomplish the right design. During prototyping, two or three variations may be tested at one time.
Many initial firearm spring designs are overstressed, approaching the limit of the spring design capability because designers want to do more than is theoretically possible with the extremely small amount of space assigned for the spring. With an overstressed design, the spring will take a permanent set, losing its length and load. To guard against this, significant development and prototyping is necessary.
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 design process included several rounds of prototypes. The firearms manufacturer adopted the recommendation and the spring made from the chrome silicon flat wire was successful.
A successful development process is also helped by knowledge of the gun industry and experience with the interaction of parts in a weapon. Knowing the variations in how a similar part works across many different models of firearms will lend itself to the next one.
This type of knowledge is important in dealing with the many changes resulting from an important industry trend – the move towards slimmer profile, smaller weapons that are meant to be concealed. Calling on knowledge gained from designing medical and surgical tools, where there is also little room to work, CSS steers firearms customers towards design directions that can be successfully manufactured. Options include using secondary machine operations to make small features.
Another growing trend is the use of polymer in pistols for both cost and weight reasons. In this case, CSS recommended stamped front and rear struts to provide the reinforcement needed for a lighter weight firearm with no steel frame. Stamping the front and rear struts strengthen the polymer frame without the need to machine the entire front of the frame.
Manufacturing processes play an important role in part quality and consistency
It takes a great deal of technical, engineering expertise to get consistency in firearm springs and stamped parts. The 127,000th part must be just like the first, and a firearm must still work if a replacement part is inserted. CSS uses a Cpk (process capability index) to demonstrate that tolerances meet specifications for the parts critical to gun functioning. This involves taking about 30 measurements and inputting them into a software program; some dimensions are monitored directly with probes and lasers. An operator controls the system with lasers and probes, and if the process hits a point where the Cpk is outside of specifications, it shuts down until corrected.
One of the most important manufacturing processes used in developing springs and stamped parts for firearms is heat treating, which is frequently used to alter the parts’ physical properties to make the part shape and size meet specifications. Firearm manufacturers must have an excellent working knowledge of heat treating and processing to ensure that parts end up in the correct shape.
Every part has different requirements and different processes can be used for firearm part heat treating, including atmosphere heat treating in bulk or racks, and non-atmosphere heat treating. The non-atmosphere method is cheaper but it can leave scales that can be unsightly; these can be removed by a post process. The customer may choose to use a less expensive heat treat process, but later pay to clean the parts. Heat treating option selection is frequently price driven.
Heat treating can correct distortion that can occur in a spring due to the extreme stress placed on wire when forming a spring. When magazine springs are formed, they are often intentionally distorted and may not meet the print specification. To correct this, spring manufacturers can intentionally introduce stress into the springs when formed in a distorted fashion, then process the springs in an inline oven to stress relieve them. After they are batch heat-treated, they take on the correct shape.
For example, one customer wanted to use 302 stainless steel for a new gun’s magazine spring. The new weapon was to be made of all stainless steel components so it would be completely corrosion-resistant. CSS engineers determined that the original design was not acceptable for the overall requirements and that they needed more load on the spring. Engineers suggested the use of 17-7PH material, heat treated at 900° to get the required spring properties to take the higher load.
Over a six-month period, numerous prototypes were developed and tested; the design was modified and improved to get the load and compression the customer wanted. Deflection and travel tests were conducted and a short run of parts was made out of both materials to see which would work better. The customer eventually selected the 17-7PH spring; although the ultimate solution did involve a material increase, the gun was released recently to great acclaim.
Heat treating is also used for stamped parts. For example, a trigger bar may be stamped flat but can distort when heat treated; the correct heat treat process must be used to ensure the part will be the correct thickness.
An accumulation of knowledge is definitely important in the firearms industry. Manufacturing quality parts depends upon working towards a solution rather than just giving customers what is on an initial drawing.
Dale Pereira & Pete Marut are at Connecticut Spring & Stamping, a Farmington, CT company with more than 160 firearms industry customers. The company makes every manner of firearm spring, as well as slide-stop assemblies, interceptor latches, magazine tube assemblies, saddle rings, speed latches, action bars, sights and trigger bars for many different hand guns and rifles, including the AR15/M16 rifle platform.