Springs: A Primer

The fine double gun is one of the few mechanical devices that has largely resisted the advances of modern technology. Though manufacturing processes have evolved, shotguns have retained the same basic attributes for well over a century, and the mechanisms at play inside them are still gestures of exquisite engineering driven by age-old forces. Beneath the surface, a shotgun is composed of pieces with such treacly names as “tumblers” and “bridles” and “posts” and “sears.” These components work in concert to keep a gun shut tight enough that it might deliver a blow to a cartridge primer, make a big bang and eject a spent hull while simultaneously cocking the hammers once again. Though the surfaces of a fine double may be dressed up with engraving, jeweling, bluing and case-hardening, at root the practical beauty of a shotgun is the platform it creates for a series of mechanical functions, each one made animate in metal and wood and each one a player in an action that (hopefully) results in a crushed clay or a bird in the hand. The tie that binds one piece to the next—the through-line that makes a shotgun mechanically and dynamically complete—is the spring.

Clockwise from top left: a Boss toplever return spring with a broken leg; a newly made and fitted Boss toplever return spring; a broken Woodward snap-action underlever return spring beside a newly made spring; the newly made return spring installed (see arrow) in the Woodward’s triggerplate. (Note that the Boss spring is a V spring, and the Woodward is a leaf spring.) Photos by Delbert Whitman Jr.

Springs come in various shapes and sizes, but they share the virtue of storing and releasing energy. Most of us are familiar with the coil springs that inhabit pogo sticks and ballpoint pens, and such springs can be found in some classic American and British doubles. More commonly, beefed-up versions of coil springs find a place in a host of contemporary shotguns—notably most of the over/unders that have followed in the wake of the Browning Superposed. Much is made about the durability of coil springs, but they do have drawbacks, mainly that they require structure—either a central post or some cylindrical body—to keep their movement linear. This can cost precious space in the minimalist internals of a gun. Though some say that coil springs have the advantage of continuing to function to a degree when broken, in practice a broken spring reveals itself in obvious terms quickly and in the worst case by depositing a fractured piece of metal debris into the closing mechanism of the gun. More common, at least in higher-grade break-action guns, are leaf springs and V springs, which can be fit into small spaces inside the lockwork and can deliver a significant amount of “work” in a small linear space.

Unlike coil springs, leaf and V springs are pieces of metal that become compressed into or out of a bend rather than compressed into a linear coil before being allowed to rebound. Virtually every action that a shotgun performs begins either with the compression of a V or leaf spring or the release of said compression and a resulting transfer of energy. Leaf and V springs are employed widely in best guns—sidelock and triggerplate guns especially—as mainsprings, toplever springs and ejector springs. The use of the V or leaf type really hinges upon the space available. A leaf spring does not have the defining radical bend of a V spring and therefore can deliver a significant amount of energy without involving the descriptive kink, or V. This in turn makes the leaf spring stronger, nominally anyway, but requires sufficient space in the gun’s internals to do the necessary work. Where space is an issue, such as in the Holland & Holland sidelock mechanism, the leaf is, for conversation’s sake, folded in half. The resulting V spring takes up far less linear space.

No two springs are exactly alike, and few are readily available for plug-and-play application in the repair or manufacture of a fine double. For this reason, a gunmaker must possess a comprehensive understanding of springs and their manufacture, largely because springs are made to order and broken springs are replaced and/or replicated by a process that involves extensive handwork. Those who build or fix best guns or classic guns will spend a significant portion of their careers fabricating springs out of raw metal, noting that the process requires precise cutting, shaping and fitting of a static part but also a solid understanding of the degree to which that part will compress and rebound under a pre-determined degree of pressure. In essence, the gunmaker must understand principles of construction, metallurgy and physics and must understand how those principles can be applied to effectively store and deliver energy. Add to these considerations the fact that a spring must remain tough and durable over a lifetime of heavy use (perhaps abuse) and that a spring under compression invariably will take a degree of bend or set, and you have a better understanding of the complex alchemy that is springmaking.

In discussing springs and how they are constructed, let’s start with some universal truths. Leaf and V springs are made from a block or bar of stock spring steel, and steel is a compound of iron and carbon. Most spring steel employed by author Del Whitman is nominally 1095, which refers to a steel alloy composed of .95% (approximately 1%) carbon (though an alloy of lower carbon content traditionally has been used). This alloy has a higher concentration of carbon than standard sheet steel, which allows it to take the heat-treating required to make a traditional spring sufficiently hard while remaining elastic. In layman’s terms, one might imagine the iron molecules in spring steel as ball bearings and the carbon as a medium like clay. The ball bearings are scattered throughout the lump of clay, and with heating and cooling, the clay expands and contracts, becoming either stiff or slack. This expansion and contraction allows the ball bearings that are suspended in the clay to move apart with heat and to pull tighter together once cooled.

In most cases when a gunmaker sets out to make a spring, he or she first measures, cuts, tapers, bends and files/polishes the steel to its approximate finished dimensions. (Note that this process alone requires a gunmaker to create a complex shape in metal, doing so with enough precision that the spring fits into the tight confines of the lockwork or inletted stock.) Once that process is complete, the spring needs to be hardened and tempered.

Inherently, springs need to be hard so that they are rigid enough to retain their shape rather than taking a bend or set. Heat-treating is the process by which ferric metal is hardened and made resistant to bending. In heat-treating, 1095 spring steel is heated to its most excited state (to a temperature around 1650°), at which point the carbon (or clay) relaxes, allowing the iron molecules (or ball bearings) to spread out. The steel is then cooled by quenching in oil, thereby contracting the bond between iron and carbon (or contracting the clay and pulling the ball bearings back together). This contraction makes the steel hard, and the degree of hardness, measured as a Rockwell figure, correlates directly to the intended function of the spring and the degree to which it may be compressed. Typical spring steel can be hardened only to a rating of 60 Rockwell, which is too hard to file and very brittle. A 46 Rockwell or so rating is far more common and more receptive to the final shaping and polishing that is often required after heat treating and tempering is done.

Now that the heat-treated spring is quite hard but brittle, its composition must be made more elastic. The process of making a spring “springy” rather than brittle is known as tempering. Tempering requires the gunmaker to heat the spring again to a point at which those molecular bonds relax enough that they won’t fracture under pressure. Some hardness is sacrificed, but elasticity is gained, and the resulting “springiness” is what gunmakers call “toughness.” When a spring is not sufficiently tempered, it will remain brittle and likely break. When a spring is over-tempered or under-hardened, it will be too soft and take a set or an unwanted bend rather than storing energy. A properly tuned spring will, under first compression, take a tiny set but will spring back readily and will have sufficient toughness to do so reliably for a lifetime.

Clockwise from top left: A broken mainspring (crack visible near bend) and a roughly forged spring bent in half to form a "V"; the roughly forged spring with legs bent back open to form the proper curve; author Del Whitman using an oxy-acetylene torch to heat a newly made spring to critical temperature before quenching it in oil to harden it; an assembled lock with a newly made mainspring installed (the broken spring is in the foreground). Photos by Delbert Whitman Jr.

So why do springs fail? Springs can fail from over-use. For example, continued pressure applied to a toplever spring by manual compression will look for a weak point through which to release. As a result, inconsistencies in hundred-year-old steel—even microscopic flaws—are the culprits of many broken toplever springs. Moreover, unrelated damage and/or wear can result in broken bits of metal or shards of wood rattling around in a shotgun’s guts, and this debris poses the risk of interrupting the intended travel of the spring between compression and release. This proverbial “wrench in the works” often reduces tolerances or occupies space that in turn forces the spring to over-flex, creating untenable pressure that can fracture a spring.

A gun with a broken spring is no gun at all, and immediate attention is required. Perhaps the most remarkable facet of spring fabrication and replacement lies in the incredible precision required of springs to function effectively and the implicit imprecision that goes into their manufacture. Even with the most consistent steels, variability in heating time and temperature, cooling time and temperature, degrees of set and levels of toughness all lie outside the absolute command of mere mortals, especially gunmakers working in rustic shops with centuries-old tools. In fabricating springs, therefore, the gunmaker or gunsmith must rely on the subtleties of intuition accumulated over hours and years. Most gunmakers or ’smiths worth their salt have broken, bent or mis-fit a good number of springs and discarded a good number more in-process before attaining any degree of consistency. In watching a gunmaker work with springs, it becomes abundantly clear that there is nearly as much precision measuring and temperature-taking as there is licking a finger and holding it up to the wind; a good gunmaker often accepts or rejects a spring by compressing it, feeling it rebound and deciding if the resulting sensation is “right.”

All of this discussion of springs proves, as mentioned, that gunmakers are equal part artist, craftsperson, engineer and alchemist. Which is what makes gun craft so entirely fascinating and so worthy of reverence. Perhaps gunmakers, like the fine guns they make, resist the advances of modern technology. That fact alone seems worthy of celebration.

Delbert Whitman Jr. lives near Traverse City, Michigan, and is a professional gunsmith specializing in repair, restoration, stockmaking and engraving. Reid Bryant is an Editor at Large for Shooting Sportsman.

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