Quackenbush Air Guns

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This page contains these articles:
   Plain Carbon Steel for Airguns
   Steel for Airguns
   What is a Billet
   Understanding Tensile Strength
   Airgun Reservoir Threads

Plain Carbon Steel for Airguns

During discussions about airguns at the Little Rock Airgun Show, many people talked about some of the things they were making.  The names of some of the steel they were going to use were 8620, 4140 (chrome-moly) *1, S7 (a high shock resistant tool steel), and others.  There are two things that I dislike about this idea that you have to use alloy steel to make airgun parts.  The first is that much of the attributes of the alloy steel mentioned are the steel’s characteristics AFTER heat treatment, and not the condition of the steel as they get it or would use it.    The second is that they would have to acquire this alloy steel and then have the extra difficulty of the low machinability.  All of this making the project take longer and inducing difficulties, prolonging the project’s time.

Plain carbon steels can be used for making airguns.  Plain carbon steel’s major alloying element is carbon.*2  The SAE number designation begins with the first of four digits to be 10, and the second two digits to be the carbon content in the percentage of 1%.  So a low carbon steel would be written as 1018.

     Low carbon steel, also called mild steel, has a carbon range of .05 to .30.  The low and high of this range of steel would be written 1005 and 1030.  This low carbon content steel cannot be hardened by applying heat and quenching.  These steels would require a higher carbon content in order to be hardened and the only way to change a low carbon steel to a higher carbon steel is to infuse carbon into the surface layer (because you can’t make it go any deeper than that), which would be called case hardening.  A spring gun’s end cap and barrel block would satisfactorily be made of 1018.   A spring gun’s tube would be made of 1020 or 1026.  This tube is readily available, is commonly used for pneumatic and hydraulic cylinders, isn’t crack sensitive *3 and is more fatigue resistant than some alloy steels.  To make the barrel, you can use 1018 steel. It is common as dirt and more than tough enough and wear resistant for lead pellets.  If you were hammer forging/cold forming the barrel, 1018 would work just fine.  It doesn’t exhibit cold shortness, which means it would flow/form very well without having to be heated.  These same characteristics are useful for making other parts that would be formed without having to heat the part red hot to bend it.

     Medium carbon steel has a carbon range above .30 to .60 and is written as 1030 and 1060.  This steel can be directly hardened by heating and quenching in water.  This steel is what you would use for link pins and trigger parts.  One of the commonly available steels for doing this with is 1045.

     High carbon steel has a carbon range above .60 to 2.0.  Although carbon is soluble in steel, up to 2%, commonly available high carbon steels usually don’t exceed 1%.  This steel can be directly hardened by heating and quenching, but usually requires a slower quench such as brine water or light oil, and tempering.  This higher carbon steel would be used for the main spring.  The steel for this is 1095.  High carbon steels, like 1095, will easily loose their carbon content near the surface when heat-treated, unless done in a vacuum or atmosphere controlled furnace.  The heat treat process would burn the carbon out of the surface layer, leaving a rough surface that would promote cracking of the spring.  Polishing the formed spring would be nearly impossible.  That’s why springs are made of tempered and polished spring wire.  You purchase the wire already tempered and polished, and wind it into the spring yourself.

     Now I’m not advocating that you have to build a gun this way.  My advice is don’t let your project be slowed down or derailed because of a dependence on the belief that you need an alloy steel to make the airgun.

     At the Little Rock Airgun Show I had seen an airgun part and the fellow said that he made it out of 4140 and was welded in part of its fabrication.  I asked him what temperature he tempered the part to and he was dumbfounded for an answer.  He hadn’t tempered it.  The thought escaped him.  The welding heat would adversely affect the 4140 alloy, making it brittle in and adjacent to the weld area.  Tempering would prevent this brittleness.  Without tempering the part is liable to break right next to the weld. 


1)     Chrome-moly is a general term.  It could be used for any of the 41 prefix steels, but depending on the carbon range of the steel it would require a different heat treat.  So if you say “chrome-moly” and you’re using a 4130 steel, so as not to confuse it with 4150, why not just say “4130”.  If you have to use the words “chrome-moly” use it after the number: “4130, chrome-moly”.  But even that is redundant because the 41 prefix designates the steel as a chrome-molybdenum type.  Thus you would be saying: “Chrome-molybdenum, .30 carbon content, chrome-moly”.

2)     Almost all steels contain manganese.  Manganese is added as a de-oxidizer to purify the steel.  Manganese has an affect on the hardenability of the steel, but is it very small, and not to the extent that carbon does.  Other elements found in steel, but are considered to be impurities and are held to a very low amount are phosphorous, sulphur and silicon.

3)     Crack sensitive is the affect that some alloy steels, if not properly heat-treated and tempered, will crack due to fatigue or work harden in stress areas.  So if your spring gun had a pinned-in cap, the tube would start to crack around the cross pin holding the pin in.



Steel for Airguns

   When I make an airgun, I use steel.  I don't substitute aluminum, zinc die cast or brass, where steel should be used.  Just because these non-ferrous metals are softer and easier to work with is no reason to use them as a substitute for steel, when steel is best for the job.  Steel is the material of choice.  In my first vocation as a tool and die maker, I used and made everything of steel.  I have sheared, formed,  blanked, drawn, machined, cut and welded steel to make the tools of industry to shear form, blank, draw, forge, cut and weld on a mass manufacturing scale.

   For airguns, I don't use just whatever steel comes to hand.  I specify particular steels for specific uses.  The selection of a steel alloy is based on the function of the part.  Does the part have to be hardened?  Is it to be tough but ductile?  Will the machining of the steel leave a smooth finish?  Understanding the properties of steels allows the selection of a specific type that is suitable for a particular use.

   Steels have a designation number based on its elemental composition.  The Society of Automotive Engineers (SAE) has a numerical system of designation.   The SAE steels I use in making the Outlaw Rifles are:  1018, 1020, 1045, 1095, 1117, 1215, 12L14, O1 and 416 stainless.

   1045 is chosen for a part that needs to be hardened.  The primary hardening element in steel is carbon.  The last two digits of 1045 are the carbon content.  1045 has .45 of 1% carbon content; actual allowable is between 0.43 and 0.50.  Above approximately .35 carbon content can be hardened directly.  Below .35 carbon content, carbon has to be added to be able to heat treat harden it.  An example, 1018 steel has to be carborized (adding carbon) which only penetrates slightly into the surface.  The result is case hardening.  1045 has approximately .45 carbon content dispersed throughout, so it can be hardened all the way through.  Or, by flame hardening, as deep as the hardening temperature is allowed to penetrate during heating.

   1117 steel is a free machining alloy.  It will yield a smoother finish than any of the ten "10" prefix steels.  The rods and bars, zinc plated and plain, that are sold in hardware stores are most commonly 1018 steel.  As a comparison, 1018 has a tensile strength of 70/80 KSI (thousand pounds square inch) and a hardness of 80/90 on the Rockwell Hardness 'B' scale.  1117 is 80/90 KSI, slightly stronger, and at 80/90 B just as hard, but it machines almost twice as easy.  No tearing and burring of the machined surface.  This steel (1117) is widely used for barrels on .22 rimfire rifles and shotguns.  Likewise, I use this steel to make barrels for air rifles.

   Now, the lowly 1018 steel still has its place.  1018 is inexpensive and commonly available.  1018 does not exhibit cold shortness.  This is a condition where steel will fracture rather than bend.  Steels that do not have cold shortness are heated to allow for bending.  1018 is used for a shock load within its strength limit.  It will not eventually fail after repeated shock loading due to fracturing.  1018 being tough, but ductile, is the steel is use for the action lug.  This is what mounts the stock to the action and transfers the recoil to the stock.

   Stainless steel's (SS) greatest asset is its corrosion resistance.  Stainless steel is predominately iron (Fe) alloyed with the expensive elements chromium (Cr) and nickel (Ni).  For this example I divide commonly encountered stainless steels simply by magnetic and non-magnetic.  Magnetic stainless steel has SAE numbers in the 400 range and contain chromium from 11 1/2 to 18%, dependant on the type.  Non-magnetic SS has a 300 numbering range and has both chromium (15 to 26%) but also nickel (1 to 22%) as alloying elements.  

   For longevity and heavy duty use, stainless steel is the upgrade from brass in valves and sealing surfaces, as used by industry for pneumatic and hydraulic uses.  I use stainless steel for the valve body in the Outlaw rifles.  My choice is 416 stainless steel.  416 has a machinability rating the same as 1117.  Not so difficult to cut as to break taps and dull cutters.  This is important, for the valve is threaded in three locations.  The threads must be smooth to engage the mating parts.  The threads cannot be weakened by ripping or tearing of the 'V' thread.

   The valve seat surface must be machined smoothly without burring or smearing that can happen with other grades of stainless steel.  The valve seat surface is first cut in a lathe, then coined in a press (impacting the surface with a hardened steel form to compress the metal), then polished in a speed lathe, making the best surface for valve sealing.  

   Many air rifles use brass for the valve.  Typical construction has a brass valve threaded for the connection of the valve and the reservoir tube.  The commonly used SAE #360 brass has a tensile strength of 58 KSI.  This is not as strong as 416, at 80 to 100 KSI.  For this reason the brass threaded section must be longer to engage more threads.  The extra length makes for a longer path for the air charge, adds extra weight and reduces the volume of the reservoir.

   Most air rifles that use a brass valve are in the 12 to 30 ft. lbs. range.  The Brigand (.375 cal.) starts at 60 ft. lbs. and the Bandit (.50 cal.) exceeds 250 ft. lbs.  The valve area is larger and the cycling is more energetic than other pre-charge rifles.  #360 brass is softer, 78 B Rockwell, than #416 stainless steel at 90/95 B Rockwell.  For long life, #416 SS will stand up to valve seat pounding where I found #360 brass will not.  

   I do use brass.  I use brass for what it is suitable for, a bearing surface.  I use it for the 'O' ring gland nut that seals the valve stem.  
   There are other mechanical properties that I consider in steel selection.  Elastic modulus, elongation yield and Izod shock properties are beyond the scope of this writing.

   One of the nice things about steel is the surface treatment called bluing.  The steel parts are polished for a reflective surface, then immersed in a 285 degree bath of dissolved salts, which gives a black oxide rust resistant blue-black finish.  There is a cold "bluing" and I've seen it on some very expensive airguns.  The maker of the cold bluing says that the cold bluing is not as good as the hot bluing, but is a inexpensive substitute for it.  Why substitute, why not use the best to begin with?  I would rather use quality materials and processes than hyperbole.
© 2004 Dennis Quackenbush


Added Aug. 24, 2005

This was originally a post of the Airgun Forum about the misuse of the term “billet”.  This was an informational post so that aspiring machinists would use the proper terms.  It became a hot button issue, one egomaniac actually demoted himself from airgun maker to a breech maker (such as making Crosman replacement breeches) so he could claim I was picking on him.  Since so much interest was shown I decided to give it a permanent place here.

What is a Billet and Why an Airgun Doesn’t use it

A billet is a semi-finished piece of metal and they’re not used in an airgun.

Airguns are made of finished metals; bar stock (round, square or rectangular) and tubing.  All of which has been cold finished, or cold drawn, so as to have a good surface finish.

When molten metal is poured into an ingot mold, the metal is allowed to cool until it solidifies.  While it is still hot and “plastic” it is rolled to shape.  The rolling is just like using a rolling pin on biscuit dough.  The roller presses down and smoothes the surface.  On the hot ingot it is done from all sides to make a square or rectangular, in cross section, piece of metal with rounded corners, called a billet. 

A billet is a semi-finished piece of metal that has been roughly worked to shape and will go on to other finishing processes.  The billet will be re-rolled while hot to further bring it to the desired finished size.  This hot rolling has “scale”.  The scale is the decarborized metal that the carbon has been burned out of.  While the metal is red hot, its surface is in contact with the atmosphere, which burns the surface.  To get a good surface on the metal the final processes are done cold.  When done cold, there is no scale produced and the surface finish is better.  But cold rolling takes more time to do because you have to use more passes, through the rolls, to size the metal.  The rolls, used for cold rolling, have to be run at a much higher pressure to form the metal in the cold state and the rolls have to be maintained with a high polish, so as to leave a good finish on the metal.  The cold finishing of the metal is the expensive part to do, but the bar stock produced has a good surface, is dimensionally accurate and, on square and rectangular stock, has square corners.

Billets, the unfinished metal bars, are used where the manufacturers don’t want to pay the extra money for the finishing of the metal because it would not add value to the finished product.  For example: forgers use billets because every surface of the metal is reshaped in the forging process.  A race car crankshaft can be machined out of a billet because the surface finish of the billet has nothing to do with the finished product, so why pay the extra money for a good surface finish?   Custom aluminum wheels are made of billet because the finished product in no way benefits from starting with good surface finish metal.  So why increase the cost of the finished product by wrongly specifying that a cold finished metal would be your working stock?

A breech maker tried to impress people by saying that the breech was made from a billet.  The breech was made from bar stock and it’s a mistake to believe that making a breech from a roughly formed piece of metal would make a better breech.  But that’s what people claiming to make things out of billets are telling you.  When I have students in the shop, one of the things I insist on is that they learn “the language”, to know the names of things and their use.   If one doesn’t, they’re going to use the wrong terms and “impress” people with how much they don’t know.  If you don’t believe me, call your local steel warehouse and tell them you want to order a billet.

I use bar stock and drawn tubing to make airguns.  Billet steel doesn’t come in small sizes and, being a semi-finished product, it has a poor surface finish, like an “I” beam or angle iron would have.  During the discussion on the forum, Tim summed it up well by saying that the term “billet” is used more for marketing & merchandizing than for its true definition.  Scott Laughlin had good insight that currently the term "billet" is being misused, just like the term "turbo" was in the 80's.

Other terms:  Bloom, slab, sheet bar are similar to a billet in that they are semi-finished mill products of square or rectangular cross section, hot rolled from ingots, but not finished rolled so they have rounded corners.  The difference between them is their cross section area and their intended use.  These terms are used by industry and are defined by the industry organizations: ASTM (American Standards Testing & Materials), SAE (Society of Automotive Engineers), ASME , AISI (American Iron & Steel Institute) & ASM (American Society of Metals).
© 2005 Dennis Quackenbush


Airgun Reservoir Part 2: Strength of Materials, Understanding Tensile Strength

To determine the strength of materials there is a standardized test.  The subject material is formed to a standardized shape then it’s placed into a machine and is literally pulled apart. 

The first picture shows standardized test specimen sizes. 

The second picture shows a test sample after being pulled apart.  This test yields information of a multitude of characteristics of the material that was tested.  The information, such as the number of pounds per square inch for tensile strength, is the strength of the size and shape of the sample used in the test.  This number is not directly transferable to any other shape of the same material except when used in the proper formula for determining the strength of the item you are working with.

First is the tensile strength; that is the amount of force needed for the material to break.

Elastic limit is the amount of force applied, that when released, the material will not return to its original form.  So below the elastic limit the material can take that amount of load and, when the load is removed, recover.

Yield strength is the other side of the elastic limit.  At this level of applied force permanent deformation takes place.  This is where the metal stretches and does not return to its original shape.

Steel companies usually also list these two other types of information to ductile materials:

Elongation: as a percentage of how far the material stretched between its yield point and its tensile strength.

Reduction of area: as a percentage of how much the material thinned out (stretching by thinning) prior to breaking.

The tensile strength figure arrived at is the ultimate strength which when an engineer uses that figure it is formulaic.  The tensile strength listing is the strength of the test piece.  This test piece data is then used in engineering formulas, taking into account the materials shape and thickness.  The tensile strength figure is not an end-all in itself and should not be used as the actual working strength of the material.  When selecting material, the engineer always leaves a safety factor where the ultimate tensile strength is reduced by 20-50% as the actual working application of the material.

This picture shows material samples all of the same rated tensile strength, but their applications are entirely different.
1626.jpg (69130 bytes)

The first one on the left is a solid bar.  It has the same rated strength as the others, but since it has no cavity it’s unusable as an airgun reservoir.

The tubing, using typical minimal tensile strength, without a safety margin, would have an ultimate burst pressure of:

Second from left is .156 wall thickness.  Burst pressure would be 20280psi.

Third is .125 wall.  Burst would be 16250psi.

Fourth is .093 wall.  Burst would be 12090psi.

Fifth is .065 wall.  Burst would be 8450psi.

And the one on the far right is .032 wall.  Burst would be 1460psi.

But an engineer would not use the burst pressure for usage, he would select the yield strength of the material and include a 20% safety factor on that and then would calculate its working pressure to have a multiple safe overpressure.  As an example, this would make the .065 wall thickness tube’s working pressure to be 2288psi.

You can find the minimum wall thickness of a tube that would hold the pressure, plus the safety factor of the working pressure, that you want to use.  But is it thick enough to be threaded?  Even if it were not threaded and had a cross-pinned plug end, is it strong enough to resist the end pressure and not tear off the end of the tube with the plug?

Yield strength is to be used in the strength calculation.   In the event of failure would you want it to fail “soft”, or resist failure to the highest degree and fail “hard”?  A theoretical example is if you were to charge the reservoir rapidly through a small orifice (creating heat), igniting any oil, plastic valve or seal material inside of the reservoir.  In this example you would have a very rapid pressure spike.  If you chose a material that has a greater amount of pressure difference between yield and tensile, where the threaded or pinned plug would be, the metal would move out of the way (expanding), creating a leak, which would be a pressure drop (like a safety valve).  With some pressure drop, the pressure spike may not exceed the tensile strength of the reservoir.  In this case you have a no longer serviceable reservoir, but you have averted a potentially worse disaster, the “soft” failure.

The opposite example is using a material that is strong and would contain the pressure spike. The error is to make a lightweight reservoir using material that has a high yield and tensile strength.  During the pressure spike, the pressure builds up to the point of yield, but the yield is high enough that it can’t yield fast enough to let off pressure.  Therefore the pressure spike could exceed the tensile strength of the material.  Possible failures are: the plug end or the connection end would separate, or if the ends held, the reservoir tube would split longitudinally releasing all the combustion gasses at once, the “hard” failure.  If size and weight were not a concern, it could easily be done by just building a massive reservoir, but then it would be too heavy to be used on an airgun. 

Hydraulic tubing is the opposite of the above, where the tensile and the yield are too far apart.  An engineer uses the yield strength for determining a pressure vessel.  In Airgun Reservoir Part 1 I mentioned that there was no engineering recommendation for using hydraulic tubing as a pressure vessel and that’s because it’s annealed (made softer/more ductile).  It is annealed so as to be formed, bent, flared and crimped without cracking.  This tubing still has the tensile strength of 55,000psi, but the yield strength is only 25,000psi.  This is why anyone wanting to build an airgun should have reference material, such as Paper Tools for Making Airguns on the "In the Shop" page.   The comparison is if you were using DOM tubing, 1” outside diameter with .093 wall thickness, the working pressure would be 3273psi, but using the same size tubing out of hydraulic tubing the working pressure would be only 1488psi.   The 1” hydraulic tubing’s maximum, before yield, is 3720psi.  720psi more than 3000psi fill, is not enough of a safety factor.  This is the difficulty of inferred engineering.  The inference is “if it’s good enough for hydraulic fluid, why isn’t it good enough for an airgun reservoir”.  Why isn’t it good enough for an airgun reservoir?  Because as manufactured tubing, it was not intended as a pressure vessel and if you didn’t use 25,000psi for your calculation, and relied on tensile strength of 55,000psi, you only have half of the strength you thought you had.  Using hydraulic tubing, 1” diameter, to be used as a portable pressure vessel (an airgun reservoir), with a working pressure of 3000psi, and a 2½ times safety margin, you would need a wall thickness of .188 (3/16 of an inch).

Tensile strength is the point where the material fails.  Yield strength is the point where the material gives.  For an airgun reservoir you have to calculate the yield point, where it will give.  To use tensile strength only is courting disaster.

Steel tubing is what I had in mind when I wrote this.

The basis of the information is from steel manufacturers and “Metallurgy Theory and Practice” by Dell K. Allen.

Any errors will be corrected, please inform me.



Airgun Reservoir Part 3:  Threading

The strength of a thread is its root, which is the base of the thread (A).  Since the thread form (shape) is a 60° ‘V’, if you make the base wider you proportionally make the thread depth greater, and because the root is wider, you end up with less threads per inch (a coarser thread).

How many threads per inch should be used?  The material used dictates threads per inch.  The softer the material, the wider the base needs to be to give it the strength necessary, so you would have a coarser thread.  Brass would require a coarser thread than steel and aluminum would require a coarser thread than brass.  As a comparison, if the thread depth (C) were 16 threads per inch for a soft material, then the shallower depth (B) would be 20 threads per inch, as you might use for steel.  The machinist rule is: the softer the material, the coarser the thread.

An example is using a 1” outside diameter steel tube and a 7/8 diameter thread, but changing from a steel end piece to an aluminum end piece.  If an aluminum end piece is used in order to save weight, the weight savings are lost because you would need a thicker walled tube (D).  A thicker walled tube is needed to accommodate the additional thread depth of the coarser thread that is required for use with aluminum.   A thinner walled tube (E) would be used if you were using a steel end piece; the difference being the extra depth of the thread has to be added to the inside diameter of the tube for the threads to engage in to.

Just as thread depth is important to the pressure vessel, so is the method chosen to do the threading.  If a tap is to be used, you would want to use a “plug” configuration tap, so that the threads beyond those that are being used to make the connection run out (end) very quickly.  Very few threads should be beyond what you need because the bottom of the ‘V’, to the outside diameter of the tube, is the effective wall thickness.  Excessive threads would weaken the tube.

If the tube is single point threaded, the standard practice for threading is to cut a recess (B) at the length into the tube that the thread is to be, and the cutter cuts from the inside of the tube out.  This works just fine for a mechanical fastener in most instances, but it is totally wrong for use as a pressure vessel.  The recess (depth D), which is cut to below the bottom of the thread depth (C) to allow for clearance of the cutter to be set inside the tube, is now the thinnest part of the tube.  So it doesn’t matter how thick the wall is, even it it’s a 1/8” wall (A), the recess (B) is now the thinnest, and weakest, part of the tube.  (That's why I say some custom guns are dangerous.)  Consequently, the tube should be rated for pressure at the wall thickness of B.  The tube is further weakened because of the right angle cut on the inside of the recess, which is a stress riser and the most likely spot for this tube to fail.

To avoid this amateur machining mistake, you would thread into the tube just as if you were externally threading to a shoulder.  When threading to a shoulder, just as the tool is approaching the shoulder it is withdrawn from the cut.  When internally threading, you would thread to the desired length and then back out the tool from the cut, leaving the same type of thread that a plug tap would leave. 

The above is much more understandable to somebody who has already done some threading.  I invite questions so as to spur thought and the answers to help to bring clarity.

Also see Making Airgun Barrels, go to the home page and click on the box "Making Airgun Barrels".