The How and Why of the Rising Bite

 A reader recently asked about the benefits of the rising bite and how it works.  I thought that the answer may be of interest generally, hence, its posting here.

Hello D****l,

I appreciate the compliment but I think of myself as much more a student than an expert (there are enough "experts" in this field already).

The rising bite (Rigby and Bissel's patent) is a so-called "third fastener" in which a vertically-oriented bolt engages a horizontally-oriented, "horseshoe" shaped rib extension.  What this type of fastener provides, that no other "third bite" does, is positive axial restraint of the breech.  Meaning, that it prevents the barrels and standing breech from moving "fore and aft" (in opposite directions, due to breech thrust) during firing. 

Most break-action designs only restrain the barrels in the vertical plane and, while this keeps the barrels from pivoting open during firing, it also allows the standing breech to deflect axially, since this axial deflection is resisted only by the mass and strength of the material at the juncture of the standing breech and the frame flats.  This is the reason that double rifles usually exhibit "bolsters" of added material at this juncture.  Unfortunately, many of these bolsters are little more than ornamental by design.  

 

Here is an example of the value of those bolsters, in a design that is only underbolted and fit "on the circle":

 

Many third bites, such as Purdey's and Holland& Holland's hidden third bites, also allow this deflection to occur due to the fact that these "hidden bites" also act to restrain the barrels only in the vertical plane.  The so-called "doll's head" rib extensions are supposed to prevent this axial deflection but, with the exception of Westley Richard's bolted doll's head, all fail in this regard.  It must be noted here that Westley Richard's bolted doll's head was their primary (and only) fastener for quite some time and they later added the underbolt in order to make the gun have three bites, so WR's doll's head is not technically a "third bite".  The Greener-type crossbolt has the potential to prevent this axial movement but only if fitted properly, which most are not.  In fact, in most guns (other than Greeners) that employ this type of third bite, the bolt makes little, if any, contact with the rib extension.

The rising bite demands appreciable skill in its fitting during manufacture.  The underbolt and the vertical bolt must fully engage their bites when closed, and the linkage that connects them must be designed so that both bolts clear their respective bites at the same time, in order to allow the barrels to open, while also translating the underbolt's horizontal movement to vertical movement of the rising bolt.  The added time in design and manufacture obviously adds to the cost, and this is probably the primary reason that it's not seen more often.

All of the above applies to single-barrel and side-by-side guns, whose frames are inherently quite flexible.  Over/unders (especially a Boss-type) are a bit of a different story since their frames are much stiffer (more resistant to deflection), due to their frame height and the deep sides of the frame acting as shear webs.  The rising bite in the Boss guns is a very technically interesting design but, owing to the frame's inherent stiffness, it is little more than technical curiosa.

The over/under's inherent stiffness in the vertical plane, not only in the frame but also in the barrels, is one of the reasons that the O/U is technically superior to the SxS. 

I hope this helps.  If you have any further questions, feel free to ask.

Dewey

A New Series (Maybe)

 I think I'll call it "Stupid Shit You Read On Gun Forums".  

Internet forums, and gun forums in particular, continue to prove the validity of Dunning and Kruger's studies.  They remain places where people who couldn't find their own ass using both hands spout "expertise" on subjects about which they know little to nothing.

Our first installment concerns hinge pins and barrel hooks.  The apparent conventional "wisdom" is that the hook is what wears, never the pin, because the pin is "hardened" and the hook "is not".  This "wisdom" disregards the fact that the hook is not made of the same material as the hinge pin, and, that the "hardening" of the hinge pin is usually of negligible value.

 Part and parcel of this wisdom, is the idea that replacing the hinge pin in order to correct an off-face condition is hogwash: "Just weld up the hook and refit" (usually with a Dremel) is the correct course of action, according to the terminally ignorant.  How does one weld up the hook of a Model 21 when it's off the face (sorry fan-boys, it does happen)?  What about welding the hook of chopper lump barrels (which have a braze seam running right down the center of the lump)?  I have seen both of the above examples welded by gun plumbers, both American and English, and it's not pretty.

The hinge pin in most break action guns is made of the same (or similar) material as the frame in which it resides.  That material is a low-carbon (sometimes very low), plain steel.  These steels are non-hardenable on their own due to their low carbon content and so are (in this application) surface hardened (case hardened).  Without getting into a long-winded dissertation on the subject, case hardening is a process whereby extra carbon is added to the steel at the surface and, to an extent, below the surface.  This makes the surface effectively hardenable.  Case hardening consists of numerous steps, the first (assuming a virgin part) is carburizing, wherein the carbon is added to the surface via a combination of heat and an external carbon source.  The second step is the actual hardening,  which consists of heating and quenching the part, so as to transform the now more carbon-rich surface (the steel below the surface remains unhardened) and the third step (usually ignored by gun manufacturers) is tempering of the part.  Depending upon certain variables, the hard "case" that forms on the part can be relatively deep (.060" or more). 

Gun manufacturers, for reasons unknown to me, tend to skip the individual first carburizing step and combine it with the hardening step.  While this does save time, it also results in a much thinner "case".  The hard surface on a frame rarely exceeds a few thousandths, often being .005" or less, usually much, much less.  This seems to be because case hardening is used in the gun world as more of an ornamental "finish" than a functional surface treatment.  Now, with a hard skin that thin, one might surmise that the soft steel below would be free to deform just as easily as without the hardened layer.  One would be correct.  Once the very thin hardened layer wears off (or through), wear accelerates rapidly.  This is especially true in American doubles because the vast majority of those use the hinge pin as a structural element, meaning it reacts the forces generated upon firing, rather than being only a pivot around which the barrels rotate to open or close.  In simpler terms, when these guns are fired, the hinge pin is what keeps the frame from departing to the rear.  When the gun is fired, the hinge experiences the same force that the breechface does, but in the opposite direction and over a much smaller area.  This force is also offset to one side or the other, depending upon whether it is the right or left barrel that is fired, causing the barrels to try and yaw about the hinge.  This causes deflection/deformation of the pin, which accelerates wear of the very thin hard skin, because the hard skin is much less flexible than the steel beneath.

 

The fact is that the internet experts are wrong, again, as usual.  In American designs it IS usually the pin that wears and replacing the pin IS the only correct remedy.  Now, correct means different things to different people but I prefer the dictionary's definition of most words.  Below are examples from this very blog that illustrate the point.  I've done many, many more prior to starting the blog and on guns that simply were not interesting enough to make it here.  Also, contrary to what the "experts" say, a new hinge pin, when done properly, does not "stick out like a sore thumb".  They are, in fact, indistinguishable from original.

Ithaca NID

Barovnik

L.C. Smith

Fox SBT

 Fox Sterlingworth

Now, given that the hinge pin is worn, and the worn area is almost never a perfect arc, how does one weld and refit the barrel hook so that it maintains full contact while traveling around the worn pin?  Answer: It can't be done.

Spring Has Sprung

So it's as good a time as any to discuss those flexible, boingey bits, without which guns couldn't function.  I'll cover each of the different types of spring most commonly found in various gun designs, the materials that they're made from, their design and application.  I will not get into any of the non-ferrous alloys such as stainless steels, copper alloys or brass alloys since they, for the most part, are uncommon in gun mechanisms.  For the same reason, there will be no discussion of the more "exotic" materials from which springs have been and are made such as wood and fiberglass.  Belleville and air springs will also not be discussed, since their appearance in firearms is quite rare.  The explanations herein will be very simplified for the sake of brevity and to prevent any deaths due to terminal boredom.

Springs in gun mechanisms serve to return a part or parts to their original position by releasing the potential energy that is stored within the spring, when it is deflected by the movement of the part that it drives.  In simple terms, this happens because the amount of deflection in the spring remains well within the elastic range of the spring's material.  This range will vary based on such variables as the material from which the spring is made, the heat treatment of the material and the actual design of the spring.  Depending upon those variables, if the spring were loaded beyond its elastic range, it may simply deform or it may actually fracture.  Most leaf and "V" springs in gun-specific applications are made from high-carbon plain steels with a carbon content of 0.70 to 0.95 percent.  Coil springs in gun applications are generally made from music wire, which has a carbon content of 0.80 to 0.95 percent and very high tensile strength, which comes from the cold drawing process by which music wire is formed.

Generally, the springs found in firearms will experience either of two types of stress:

bending (as a beam)

or torsional (twisting).

Which type is not always intuitively obvious which you will soon see.

The term "spring rate" will also come up.  What this means is that for X amount of force applied, the spring will deflect X amount.  For example, assume a straight-rate coil spring of 10 pounds per inch.  What this means is that 10 pounds of force applied to the spring will cause it to deflect 1 inch, 20 pounds will be 2 inches, 30 pounds will be 3 inches and so on in that fashion.  That would be what is referred to as a "straight-rate" spring, easily recognized by its equally spaced coils.  There are also what are referred to as "variable-rate" springs, easily identified by their more closely spaced coils at one end.  In a variable-rate spring, the more closely spaced coils make up the softer portion of the spring and when those closely spaced coils stack solid, then the rest of the spring comes into play.  The reason that the area of more closely spaced coils is "softer" (it has a lower rate) is because that area of the spring is actually longer, which is why there are more coils and why they must be more closely spaced.  Variable-rate springs are rarely found factory installed.  They are common in the aftermarket as recoil springs for semiauto pistols.  Their value in these applications is dubious.

You may have noticed that dashed line in the center of the beam in the first drawing.  It is what is known as the neutral axis, which is the cross-sectional point where the opposing forces of bending (or twisting, in a torsion bar) change direction and cancel each other.  In the case of the beam in the following drawing, the upper surface is experiencing compression and the lower surface is experiencing tension.  Since these forces cancel each other out at the NA, it should be obvious that they are greatest at the outer surfaces and diminish toward the NA.  The NA is not always at the centerline of the stressed portion of a beam or spring.  It will coincide with the centerline only in a beam of symmetrical cross-section.  Calculating the NA for a non-symmetrical part is a bit beyond the scope of a blog post but it generally moves toward the more massive side.  It should also be obvious, now that you know this, why tool marks (especially across the surface) would shorten the lifespan of a leaf spring.

Now, on to different types of springs:

The torsion bar, not to be confused with a torsion spring:  A torsion bar is a spring that acts by being twisted about its long axis when a torque force is applied.  For a given material, diameter and load, the longer the torsion bar, the lower its spring rate will be.  This means that the longer it is, the more it will flex under a given load.  Torsion bars are not commonly encountered in firearms but understanding its function is essential because it is identical to its first cousin, the coil spring.

The coil spring:  The single most commonly used type of spring in all firearms is the coil spring.  They will be found in both compression and extension types.  It is commonly assumed that when a coil spring is compressed (or extended), that the coils bend, this is not the case.  When a coil spring is deflected, the entire spring (assuming a straight-rate) experiences torsional strain through the cross-section of the wire diameter, throughout its entire length.  This is why a coil spring gets shorter when compressed (or longer when extended).  Take a torsion bar and wind it perpendicular to its long axis into a spiral, you now have a coil spring.  As with the torsion bar, for a given diameter and material, the more coils, the lower the spring rate (it will be softer).  The coil spring has the possible advantage of still functioning even when broken, assuming it is sufficiently guided, either internally (with a guide rod) or externally (in a tunnel), and the broken ends do not "thread together" and induce coil bind sufficient to restrict movement of the mechanism.  Coil springs that have a uniform coil pitch are straight-rate springs while coil springs that have an unequal coil pitch are of variable rate.

The leaf spring:  This type of spring is commonly used as the mainspring (hammer spring) in many repeating rifles, single shot rifles and shotguns and a number of revolvers.  It is basically a deflecting beam.  It is invariably tapered as well, in either plan view or elevation, sometimes both, with the thickest point being at the anchor.  The reason for the taper is to load as much of the free length as uniformly as possible, without stress concentrations that would affect its rate and service life.

The "V" spring:  Maybe the second most common type of spring in the gun world and the single most common where my work is concerned.  In most (non-American) doubles, the hammers, ejectors and toplevers are powered by springs of this type.  It's basically two opposing leaf springs, joined at the fat end.  The limbs always work opposite each other.

The final commonly encountered spring type that I'll touch on here is the torsion spring.  Contrary to its name (and non-intuitively), it does not work in torsion at all.  Its name is derived from the way in which it drives whatever part it is driving.  That is, around a shaft or pivot pin.  This type of spring experiences beam-type bending stresses, from one end, around all of its coils and all the way to its other end.  For a given wire diameter and number of coils, the longer the "legs", the lower the rate.  Similarly, for a given wire diameter and leg length, the more coils, the lower the rate.  This is because in both cases the spring is longer.

Now that the boring stuff is out of the way, let us debunk some more "gun-world lore" because, like it or not, the gun world is not exempt from the laws of physics.

Cutting springs to make them softer:  Shortening the length of any spring INCREASES its rate.  That means it is stiffer, NOT softer.  "Bullshit!" cries the kitchen table trigger jobber, "I cut the rebound spring in all of my Smith action jobs and it works."  No Cleetus, it doesn't.  What happens is that, by cutting the spring, the installed preload is less because of the spring's now-shorter free length.  It therefore exerts less pressure when installed and because the spring is now shorter, it also does not compress as far during its cycle.  The only thing you've lessened is the reliability of the trigger return.

If the logic of cutting springs were sound, the short piece of spring that was cut off would be much softer that the rest of the spring.  This is easily proven to be false:  Take an old 1911 recoil spring (or any coil spring) and cut 4 coils off of it.  Now try to compress those 4 coils.  Let me know how that works out for you.

Leaf vs. Coil Mainsprings:  If I had a buck for every word of flowery prose written by gunwriters who've never competently operated a screwdriver about the virtues of V springs over coils, I'd be pretty well off.  Apparently, V springs are "snappier" (whatever the hell that means), faster, stronger, less filling, taste better and are generally superior in every way to coil springs.  It is a bit hard to argue while being intellectually honest, since (with one exception) there are no guns that are identical in design except for their form of mainspring.  With so many differing designs and lockwork geometry encountered, an apples-to-apples comparison isn't really possible.  It's a bit like arguing that purple is better than Ferrari.  The above referenced exception to this case is The Perazzi MX-series over/under.
These guns utilize a detachable trigger group that is available in both V and coil spring form.  So in that case a direct comparison is possible. 

The mainspring's job is to accelerate the hammer with sufficient speed to deliver an impulse to the firing pin, sufficient to detonate the primer.  Ideally, this will be accomplished in the shortest possible span of time.

Let us assume that the mass of the hammers is the same for both spring types, that the energy stored in the deflected springs is the same and the hammer travel is also identical.  At that point the most important variable is the weight of the springs.  I don't mean the weight, as in the "pressure" that the springs exert, we've already assumed that that is identical.  I mean the actual physical weight of the parts themselves.  This is important because the spring, regardless of type, must not only accelerate the hammer but its own mass as well.

Obviously the spring that has more mass will be slower to accelerate, so the lighter one wins.  Is the coil spring and its guide rod (which must also be taken into account) lighter in weight than the v spring and its swivel link?  I don't know because I've never compared the weights of these parts but it seems reasonable to think so.  Let's assume just for the sake of argument that it is slightly lighter.  Can most people discern the microseconds of difference in lock time?  That seems unlikely since most shooters can't discern a 4 pound trigger pull from a 5 pound pull.  What about the much-touted "snappiness" of the v spring?  If the hammers in both cases are accelerated at a rate that differs in microseconds, how can the claim that one is "snappier" than the other stand?  The reasonable conclusion seems to be that the vast majority of shooters would never know which trigger group that they were shooting, if they didn't already know beforehand.  Now there are some firearms whose lock time is so glacially slow (Colt SAA and 1911) that it is readily apparent to the shooter, Perazzi MX guns are not among them.

Then there are those who fetishize the v spring because they imagine Luigi the white-haired gunsmith, looking over his wire-rimmed glasses, hand filing and polishing their beloved v springs to perfection before heat treating them in the last remaining cup of whale oil on the planet and finally hand polishing them again before installation.  The reality is a bit different.  Perazzi wire EDMs the v springs by the hundreds, they are mechanically polished and heat treated in huge batches and then polished again, mechanically.  Each one is identical to the last and the next.   This is how Perazzi's v springs can be so supremely reliable.  If it's as reliable as a coil spring and offers little to no tangible advantage, then why would they offer it at all?  Well, for the same reason that they still make things like barbecue-flavored potato chips and tarted-up Toyotas called "Lexus": because people will buy them. 

My verdict:  Find something more important to argue about, like how many angels can dance on the head of a pin.

Mechanical Wall Thickness Gauges Are Worthless

Traditional barrel wall thickness gauges are inherently inaccurate due to the flexibility of the barrel arbors. This includes the vaunted "English Gunmaker's Style" that uses three rods mounted in a baseplate. The reason is that the arbors are solid rods that can be 14 to 18 inches in length and no matter how stiff you might think they are, they absolutely are not. They will flex measurably at the slightest provocation. This tool serves little more purpose than to impress the ignorant onlooker. It is guesswork with a dial indicator, nothing more, and was never the best tool for the job, even though it was the best that could be hoped for at a certain time. Thankfully, that time is passed. The ONLY widely available method of absolutely accurate wall thickness measurement is through the use of ultrasonic measurement. The tool that I use is the DeFelsko UTG P1 which is capable of measurement down to .008" in steel with an accuracy of .0004" (four TEN thousandths of an inch). Yes, it's expensive because actual accuracy costs, but it's worth every penny.

Here is some proof of the flexibility of the "traditional" gauge construction. The following video shows just how much movement there is in a .625" diameter water-hardening steel rod, at 16 inches unsupported length, chucked in a 5C collet in the lathe. This setup is much more rigid than any baseplate mount. The amount of deflection (a lot) versus the force applied (very little) should illustrate why ten different people will get ten different readings of the same barrel.

"But it's used in a vertical position" you say. Think about the fact that half of the barrel length is above the uppermost end of the arbor and that the inner wall of the barrel being measured MUST contact the arbor. Do you really believe that you can hold the inner wall in contact with the arbor, while not inducing ANY side load and causing it to deflect at all?

"But it's better than nothing" you say.  No, erroneous information is worth exactly nothing.

"But so-and-so uses it and always has" you say.   Of course, it looks snazzy, it impresses those that don't know any better and it's cheap to make.  As you'd expect, it is also monumentally overpriced to buy, which further cements its "credibility" with the purchaser.

Play the video at half speed for a better look.

Sadly, it is the best of the mechanical measuring methods available.  Yet more "tradition" that is best forgotten.

A Contrarian View of Shotgun Proof

Rifles and shotguns share two traits in common:  they're both fired from the shoulder and they both go bang, beyond that, the similarities end.  Trying to shoot a rifle like a shotgun would result in misses and groups that can be measured with a tape measure, while trying to shoot a shotgun like a rifle will result in missed targets and/or birds (this obviously does not apply to "slug guns", which are effectively large-bore, low-pressure rifles).  They are often treated similarly though, as the rise of the cult of high-velocity among shotgunners illustrates, but that's a post for another time.  What I want to talk about is proof testing, its benefits as applied to rifles, and its potential detriment where shotguns are concerned.  HERESY!, the proof fetishists will no doubt exclaim but we'll look at proof "testing" coldly and rationally here.  You draw your own conclusions.

"Proofing" has tangible performance benefits where rifles are concerned and these are rooted in a nifty property of steels known as strain (or work) hardening.  Did you ever wonder why, when looking at the properties of raw materials (of the same alloy), that cold-finished material has a higher yield and ultimate strength than the hot-rolled version of the same alloy.  The answer is strain-hardening induced in the cold-worked version.  This being a mere blog, I won't get into the eye-glazing specifics, rather I will attempt a layman's explanation.  But, before we get into that, some definitions are in order because the following terms are often used interchangeably, which should never be done.
They are:
Stress = force applied
Strain = reaction to the applied force
Yield strength = the maximum stress that a material can withstand and still return to its original dimensions
Ultimate strength = the stress at which the material will fail
Elastic Deformation = the maximum strain a material can endure and return to its original form/shape/dimensions
Plastic Deformation = the strain at which the material will permanently deform

Now that that's out of the way, let's take a look at what happens to a rifle when it's fired with a higher-than-normal-service-pressure (proof) cartridge.  I'll use small round numbers for simplicity, just for illustration. The actual numbers will vary with material, dimensions, etc.

Say we have a piece of steel that has an elastic limit (yield limit) of (to use round numbers) 1000 psi.  Obviously, that means that it will return to its original form if any force of 1000 psi or less is applied.

If we apply and then remove a force of 1002 psi (0.20% beyond elastic limit), it will plastically deform, but only slightly (in fact, barely measurably) BUT, the elastic range has now been RAISED to 1002 psi.  Meaning, that it will return to its present form if a force of 1002 psi or less is applied.   The benefit is obvious as applied to locking surfaces but it's in the chamber where this phenomenon is truly valuable.

The rifle barrel, being by definition a thick-walled tube (any tube where the wall exceeds roughly 10% of I.D.), experiences this strain hardening in a very interesting and useful way. 
When the chamber expands radially during the firing of an overpressure load (proof load), the inner portion (varies with barrel metallurgy, dimensions and chamber pressure applied) will be stressed into its range of plastic deformation, but the outer portion of the barrel (due to the great wall thickness) will be still well within its elastic range because the boundary of plastic deformation is still well within the barrel's outside diameter.  What this means is that after the pressure event of proofing, the outer wall of the barrel tries to contract back to its original dimension but since the chamber has been plastically deformed, it (the chamber) will not contract, thus putting the chamber in compressive stress, which is highly desirable.  This is exactly why a button-rifled barrel shouldn't be recontoured or fluted after rifling (because it alters the compressive stresses in the barrel).

Because of this compressive stress, it will now take an even higher pressure before the barrel yields again.

This applies as long as we stay within about the first 0.20% of the plastic deformation range.

Now a shotgun barrel is different situation entirely.  Being of thin-walled construction, any "proof" cartridge capable of straining the chamber beyond its elastic limit would, almost certainly, result in a permanently bulged chamber, simply because the shotgun's barrel lacks the bulk material to contain the plastically deformed chamber within an elastically deformed "outer ring" of barrel.  Even if this were not the case, it is unlikely that a shotgun would benefit from it, due to the shotgun's low operating pressures.  In fact, I have encountered a certain classic-era American maker's guns in 16 gauge with bulged chambers on more than a few occasions (often enough to take notice of it), no doubt attributable to "proof-testing".

Where rifles are concerned, proof isn't just "testing for safety", it is part and parcel of the building of the rifle and integral to extracting the maximum performance from the system.  The steel that the rifle is made from is materially altered in a beneficial way.  Proof "testing" in shotguns seems to be just that, "testing", to see if it will blow up or not.  Shotguns are, by design, low-pressure systems (far lower than even the lowest pressure rifle cartridge) and as long as all parts of that system are designed to contain strain levels well within their materials' elastic range, are made of quality material and exhibit good workmanship, the danger of failure is remote.  Any shotgun is subject to failure due to a faulty reload or obstructed bore (as is any rifle) but such incidents fall squarely into the category of "operator error", against which there is no proof test.  Proof testing may have had some value in separating the defective parts from the good ones back in the days when metallurgy was more guesswork than science but thankfully, modern metallurgy and (material science in general) is not what it once was.

More than one perfectly serviceable shotgun has been destroyed by "reproofing".  Does this prove that the gun was unsafe to begin with?  Maybe, but  then again, maybe not.  Shotguns are often designed to be lightweight and intentionally overstressing a shotgun with forces it was never designed to cope with doesn't really "prove" anything, whether or not the gun fails catastrophically.  It is entirely conceivable that proof-testing an otherwise serviceable shotgun could induce damage that did not previously exist.  Does this prove a fault in the gun's design or materials?  Considering that a shotgun can not benefit from proofing in the way that a rifle does, it seems that proof-testing of shotguns is little more than willful abuse to gain a "peace of mind" that may well be illusory.