Making Airgun Barrels

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Airgun Barrel Material Selection

  Before you make a barrel, you must decide what to make it out of. So let's go examine some of the popular materials used for barrels.

Brass:

  Brass has good qualities for an airgun barrel. It is easy to machine and easy to rifle, because brass chips break away cleanly leaving a good surface. Brass offers a rust & corrosion resistance. On the down side, brass is heavier than steel and not as strong as steel, so the same barrel in brass would have to be thicker walled to make up for its lack of strength, but this would make a brass barrel much heavier than a steel barrel. Benjamin/Sheridan get around this using a thin barrel and soldering it to the lower tube to give it the strength it needs. Most air cane brass barrels are supported inside a steel casing. Anyone who has an older Benjamin or Sheridan knows that when the finish wears off, how would one refurbish it? Steel can be blued and when worn it can be repolished and blued again. For modern airguns, brass requires too much to design around to make it work.

Center fire rifle barrel steel, 4130/4140, chromium-molybdenum:

  This steel was chosen because it resists the hot, burning powder gasses and the wear of jacketed bullets. The addition of alloying elements, chromium, for wear and scaling resistance, and molybdenum, for toughness and strength at elevated temperatures make a fine barrel for powder burners, but it is difficult to machine. Because the machining chips do not come off cleanly, both in drilling the barrel and in cutting the rifling, is the reason why many manufacturers lap their barrels. Lapping is a method to polish out the roughness left by machining and rifling. If you have a smooth barrel, you don't have to lap it. So if they say they lap it, then the barrel was too rough to begin with. 4130/4140 is absolutely unnecessary for air gun barrels. Nothing is gained by using this steel.  Another name used for4130/4140 is "chrome-moly".  For an airgun builder to boast of using chrome-moly is the height of ridiculousness.  One of the biggest balony defense lines I've heard is the quoting of a high tensile strength, but the tensile strength quoted is the heat treated and hardened condition of the steel and that's not the condition in which it is used in the barrel.  This just means that they're using center fire rifle barrels or hydraulic tubing that is rifled.  It's convenient for them to do this this way, but you shouldn't have to pay for their lack of proper steel selection.

Stainless steels:

  Stainless steel is chosen for its corrosion and rust resistance. The grade of stainless steel most commonly used for firearms barrels is AISI #416, which is a free machining variation of #410. The main constituent in this stainless steel is chromium. Chromium improves wear resistance, and corrosion and scaling at elevated temperatures. The reason this is important for firearms, especially target rifles, is the wear that degrades accuracy is the hot, burning powder gasses eroding the throat area. The throat is the spot in the barrel where the bullet enters the rifling. Without a good start to the bullet in the bore, the gun won't be accurate. If you're going to have a barrel and expect it to last for 20,000 rounds (target rifle), you'll have to select a material that will last that long. The attributes that make stainless steel so desirable for a firearm is unnecessary in an air gun because there are no hot, burning powder gasses. The other feature that most people look to stainless steel for is rust resistance. They use the term "resistance", not "proof", because the grade of stainless steel used for rifle barrels can rust. Stainless steel barrels are not maintenance free. Just because you have a stainless steel barrel doesn't mean that you don't have to take care of it. So it would be a mistake to put a stainless steel barrel on an air gun in the belief that you can now neglect it. At the moment, I don't see where the extra expense and difficulty of machining and rifling a stainless steel barrel would be an advantage. Without the rest of the rifle's components being made of stainless steel, and having a synthetic, weather proof stock, the stainless steel barrel only is no advantage.

Tubing, plain low carbon 1020:

  There are two types of tubing that could be used for making barrels: cold, drawn seamless, or welded, drawn over mandrel (aka DOM tubing.) Both of these types of tubing have the same two liabilities: 1) low machinability, chips don't break clean, but tear leaving a rough surface. 2) stress build-up, the forming of a bar or strip of steel into a tube causes stress in the steel that is in balance, as long as it remains as it came from the tube mill. Cut a groove in it for rifling and you change the stress balance and the barrel is going to warp. So the barrel warps when you cut the first groove and now when you cut the second groove it's going to warp a different way. After doing this seven or twelve times the barrel is not uniform. As stress is relieved, some time in the future, whether it be a week, a month or a decade, the bore might become oval or bow one direction or the other. As will be explained later, button rifling tubing applies the stress equally throughout the tube. This is the method by which Crosman, Daisy, etc., rifle their thin barrels made of tubing.

  In each step of these barrel making installments I'll point out why there are too many things wrong with using cut rifling in tubing.  It doesn't matter how high a quality of tubing is claimed to be, or the type of alloy of the tubing, it is the fact that it is tubing, with its inherent stresses, is the problem.

Free machining, 1117 steel:

  This is the steel that is used for .22 rimfire barrels. Airgun barrels don't have to deal with the hot powder gasses of a .22 or the copper plated bullets. But this steel will stand up to that, which is more than asked of it as an airgun barrel. The reason I use this steel is that, as its name implies, it machines freely. The chips break cleanly, leaving a smooth surface. This is very important because the barrels are made out of solid bar stock and a hole must be drilled completely through it. It is then reamed to size before cutting the rifling. Gun drilling will be described in another installment.

  Another attribute of 1117 is that it polishes readily and can be hot, salt bath blued, which is the standard in the firearms industry.

All of the above is the reasoning that lead me to the material that I use for airgun barrels.

Airgun Barrel Processes

  I'm going to have to delve into another area of airgun barrel making. I'm going to make a division between chip making (machining) and stress in airgun barrel processes. Stress can be likened to an internal pressure in the steel. When steel is worked, by bending, forming and compressing, internal pressures build up. The concern to barrel making is the unevenness of this pressure throughout the length of the barrel and the changing, or release, of the pressure during or after the barrel is made. This is a short and not all inclusive discussion, just about how different processes effect the quality of airgun barrels.

  The basis of barrel making is that you have a bar of steel. You need to drill a straight hole through it and ream it to size, both are chip making processes. Then you need to make the rifling grooves in it. Cut Rifling is a chip making process. This process induces no stress into the barrel. But if the bar of steel was already stressed, this process would relieve stress thereby bowing the barrel during drilling or by making the bore oval, or worse, during the rifling cutting. This is why you see so many barrel makers advertise that their barrels are made out of "stress proof steel", or another name for the same thing. So in this process if you start with unstressed steel and you induce no stress during the chip making processes, your barrel won't bow or have an out of round bore.

  The first modification of this barrel making process is to substitute Button Rifling for Cut Rifling. Button Rifling is not a chip making process and to simplify its explanation, the rifling is done radially all at one time by impressing the rifling into the interior of the barrel blank's bore. This induces stress because it's a forming process that pushes the steel around. But it's done evenly around the whole bore at one time. So the stresses are in balance. Not unbalanced as if you were to do part of it at one time and then doing another part of it later. So this barrel would start as a stress proof bar, drilled and reamed, no stress chip making process and a formed rifling process that is balanced in its stress making. This makes for a straight barrel and a uniform bore.

  Then there are Cold Forged Barrels. The rifling process is done by putting a mandrel (form) with the negative form of the rifling on it. It is slid into the barrel blank and then the barrel blank is pounded from the outside to form the positive rifling into the grooves cut in the mandrel. The Cold Forging process induces a whale of a lot of stresses into the barrel, so the barrel has to be stressed relieved after rifling. And many of the barrels subsequently need to be straightened. This is a very speedy process (Cold Forging) and some firearms makers use it because they form the cartridge chamber in the barrel at the same time as rifling it. This is a big savings in time and helps to produce a less expensive barrel for the manufacturer. This barrel making process still requires a barrel blank with a hole in it, which you have to do by a chip making process. Most Cold Forged barrels need to be straightened, and bore uniformity is not always as good as a Cut or Button rifled barrel.

  A process that I'll need to mention, but it's not used in airgun barrels, is Electric Discharge Machining (EDM). This electro-chemical process can make the bore and impart the rifling at the same time. There is no stress imparted and it doesn't make chips by shearing steel but by dissolving it away in little, tiny particles. The smallest caliber I know of this being used for is 20mm. It's most popular use is 105mm and 155mm. EDM is slow, but when you can make a large bore and rifle simultaneously, with no induced stress, you can produce a straight barrel (some are 14' and longer) that only require lapping for finishing. In this way, it becomes economical.

  Rifling tubing: The tube is not made by chip making. It is formed out of flat strip or, if seamless, it is drawn tubular. Tubing has a lot of stress from the forming process, but the stresses are in balance, that's what makes it round. If you Button Rifle a tube you're going to impart stress. Button Rifling impresses the stress uniformly, so you change the stress in the tube, but it's in balance. The rifled tube remains straight and the bore is round. This barrel requires no chip making, either in the making of the bore or the rifling, and all the stresses are in balance.  
But the difficulty in using tubing us multiple.  The button is made for a particular wall thickness of tubing and can't be used on much thicker or thinner wall tubing than what it was made for.  It has a very narrow application.  Another thing is that the tubing varies in the inside diameter, because the tubing is drawn to an outside diameter specification, leaving all the plus or minus variation in the inside diameter.  Then there is the variation in the temper of the steel from lot to lot.  The variation in temper a different amount of spring back after the button has passed through it.  With these problems you could get up to .006 difference in the bore size, which would make the necessity of oddball bullet sizes.  Large barrel manufacturers order steel tubing in a large enough quantity that the steel supplier will send them steel within their specifications.  And they might even modify a rifling button to make sure it comes out to the correct size.  But they can amortize the cost of the button over 
5,000- 10,000 barrels.  Again, taking the easy way out by choosing not to drill a hole in a piece of steel, produces the inconsistencies that does not make for a good barrel.

  Cut Rifling tubing is a fool's bargain. The false economy of not having to drill a hole, and using an inexpensive, elementary rifling setup, will come back to haunt you. Mentioned above, with Button Rifling the stresses are in balance. With Cut Rifling you take a tube that is round, because the stresses are in balance, and you cut a radial groove on its inside. You've removed metal in a spiral pattern, changing the stress unevenly, so the tube is going to change its shape slightly from this cutting away of some of the stressed steel wall. Now you have to cut the rest of the rifling grooves, one at a time, changing the stress in the tube with every cut. Even if you were fortunate that the barrel was usable immediately after rifling, over time (a day, a year or a decade) the changing of the stress is going to make the bore oval, unevenly, from breech to muzzle.

  The end quality of the barrel is determined by not only the care in making the barrel, but by the process of how the barrel is made. Some are good, some are acceptable and the last one is unpredictable.

Addendum: Special cases

  I once tooled up to make 9mm polygon rifled pistol barrels for a copy of an Austrian gas delayed 9mm pistol. I used a form to shape the polygon, in a cold forming process, but instead of using a rotary swedge, like Styer, I pushed this short 5 inch length through a draw die. Because the drilled blank was so short I didn't have to worry about it bending. I could put a draw die in a hydraulic drawing press (a back geared press would probably work also) and run the arbor and the blank through the die with the ram. This was a speedy barrel making process and inexpensive to tool up for.

  Another unusual rifling adaptation was a four barrel derringer, Sharpe's style, in .22 rimfire. Rather than cutting the rifling in the steel barrel I cut the rifling in the wax form. Cutting the rifling into the wax form before casting meant that a broach could be made out of carbon steel, rather than high speed steel, and didn't have to have as many cutter teeth because the wax is soft and chip build up is not a problem. Little effort was needed to run the broach through the wax. The set up was basically a spiral cam, like a Yankee push drill, mounted in an arbor press. Now this is not match quality rifling, but was for a pocket derringer that had sights that you almost couldn't see. But the barrels had to have rifling in them because leaving it a smooth bore would have violated a 1934 NFA act , which would have made it a short barrel shotgun, now known as "any other weapon" requiring federal transfer tax and licensing.

The Barrel’s Bore

 
This is the cutting end of a gun drill. The two holes are the oil passages and the "V" section is the single flute.


Looking straight down on the gun drill. You can see that the "V" cutting edge is all on one half of the diameter of the drill.

   Not every facet of hole making will be discussed.  This is a view of some of the interesting aspects of it, not a “how-to”.  For drilling barrels you use a gun drill, rather appropriately named, but it’s whole development was for making long, straight holes for the gun making industry.  When you want to make a long, straight hole, the work is rotated and the drill is fixed.  The reason for this is the spinning part, the center, is a dead zone.  To the drill, the center of the stock is not rotating at all compared to the outside diameter of the part being rotated.  A gun drill doesn’t center itself by cutting the very center of the stock.  This is opposite of the common twist drill.  But the twist drill doesn’t cut the center of the stock, it has a “V” section that wedges the material from the center out to where the cutting flutes are.  The bigger the diameter of the twist drill, the bigger the non-cutting center section is.  That’s why with a large twist drill you would drill it first with a smaller drill (pilot drill) that is no larger than the size of the web of the larger drill.  There is a way around this part of twist drill drilling, that is to use a split point drill.  But you’re still centering on the dead zone of the rotating stock and that doesn’t work for long, deep holes.

   The gun drill is made so that it is cutting only on one side of its diameter and the “V” point is centered in the cutting side.  The cutting action is between the dead center of the stock and the diameter of the drill.  The purposeful geometry of the cutting angles of the gun drill are such that its cutting action stops if it goes off center, so that makes it stay on center.  The actual cutting head of tungsten carbide is silver soldered onto a hollow shank and one flute is formed into this hollow shank.  Cutting oil is pumped through the hollow shank at 500 to 1000psi, dependant on the cut.  The reason for high pressure is that the oil flushes out the cutting chips, because once the drill is started it’s a continuous process.  The drill doesn’t get backed out to be cleared of chips.  The cutting action starts and it is fed at a feed rate of .0002 to .0006 thousandths of an inch per revolution of the spinning stock.  The feed rate determines the size of the chip.  A higher feed rate would make a thicker chip.  The flute size of the drill is determined by the diameter of the drill.  The flute can only be about ¼ of the size of the diameter, so the smaller the hole you’re drilling, the smaller the chip has to be, so that the oil can flush the chips out through the small flute.  The range is .0002 for .177 caliber and .0006 for 72 caliber.

    When drilling a barrel the drill is undersize to leave metal for finishing by reaming.  A .308 barrel is drilled to .293 diameter.  It then gets reamed at .298 diameter and then finished reamed at .300 diameter.  The finished hole size is what dictates (within small parameters) the size of the projectile the barrel will use.  The only variation in bore size from the finished hole would be the depth of the rifling.  If you were to rifle to a depth of .003 on a side, which would make for .006 total, the bottom of the grooves in the barrel would be .306.  If you were to rifle to a depth of .004, in the same manner, the bottom of the grooves in the barrel would be .308.  The proper bullet to shoot in this barrel would be .308 diameter.

   By machining a barrel’s bore, by drilling and reaming, you make a bore size exactly what you want, not over, not under, but right on what the finished ream size is every time.  This process allows for predictability and uniformity.  Each barrel’s bore is going to be the same time after time.

   Another reason why I don’t use tubing, besides the stress problem as stated earlier, is the lack of uniformity in tubing.  The manufacturing tolerance for the inside diameter of tubing is +.000 (over) to  -.005 (under).   An example would be that you have a tube with a listed inside diameter of 5/16, which would be .312.  So according to the specifications, the bore could be no larger than .312, but could be as small as .307 (and anywhere in between.)  So you get one length of tubing to rifle and it has a .312 bore.  On another day you get another length of tubing and it has a .307 bore.  Both the sizes are within manufacturing tolerances, which are beyond your control.  So if you’re rifling the same .004 deep on a side, the first would have a bore size of .320 and the second would have a bore size of .315.  These are not standard bore sizes and bullets cannot be gotten off the shelf.  You either have to buy an expensive custom mould make or pay somebody else to make them.  By machining the bore to an industry standard bore size, bullets are available off the shelf from a myriad of makers.  And bullet molds are inexpensive and available from multiple sources.

  Again, the false economy of using tubing for a cut rifled barrel, just because it has a hole through it already, is that you have little control over the bore size.  There is no reason for oddball calibers.  You only get them through  bad compromise.

   I have both button and cut rifled barrels.  I use what I believe is appropriate for the caliber and the quantity of the barrels I need.  An example is: for a 10 meter pistol the .177 cal. barrel will be button rifled.  For the .72 cal. rifle the barrel will be cut rifled.  Cut rifling allows me to change the twist rate, for different weight bullets, within minutes.  A rifling button's twist rate can only be changed a little bit from what it was made for, so if you want to change twist rates you have to make a new rifling button.

  

Cut Versus Button Rifling
You might find some of this redundant, but they were written as stand alone articles.

   Some people call cut rifling "scratch rifling" and I've seen button rifling called "smeared".  These people think they're rather clever with their creative writing, but their derogatory words indicate their bias and that they're not being entirely truthful with you. 

   I use both cut and button rifled barrels and the decision of one over the other is based on the quantity of the type of barrels needed.  Both processes will produce barrels of quality.  If I needed a large quantity of one caliber of barrel with one rifling twist,  button rifling would be the choice.  If I needed a particular caliber of barrel, but some of the with a slow twist and the other part of them with a fast twist, I would choose to cut these barrels because the cut rifling machinery can easily be changed for the different rifling twists.  Cut rifling takes more time than button rifling, but you can't change the twist rate of a button.  Every twist rate requires a button made for that twist rate.  Because buttons are expensive to either make or buy, you're not going to have a button for a barrel that you only have a use for a few of.  It's not a choice of either/or, but the pragmatism of what it best for the job.

   When rifling was first used, barrels were made of iron and easy to cut.  It was also a time when materials were expensive and labor was cheap.  Rifling of the barrels became progressively more difficult as barrels were made of better materials, capable of being used at higher pressures and being erosion and wear resistant.  First, low and medium carbon steel were used, then manganese steel, also nickel was added for further wear resistance.  All progressively more difficult to rifle quickly and smoothly.  Today's gun barrels, made of chromium & molybdenum alloy steel,  is even more difficult to cut quickly and have a smooth as it comes from the rifling machine.  The need to produce quality barrels quickly, because now materials are cheap and labor is expensive, lead to new processes being developed.  One of the new processes is button rifling.  The actual passing of the button through the bore, making the rifling, takes less than 20 seconds and requires no secondary processes, such as lapping the barrel.  

   The improvements in metallurgy that made barrels wear resistant also makes machining the barrel more difficult.  If you can cut down on the machining of this tougher material, you save a lot of time, which means money.  Button rifling does not have to "machine" the inside of the bore to make the rifling, it forms it to the desired shape.  When I speak of "quickly" it's all comparative because you're comparing it to doing the same thing, but by a different method.  Button rifling is quicker than cutting the rifling, which means it's less expensive to make, but being done quickly doesn't make it of lower quality.  The quality is in the detail of how well you want to make the barrel.  How much attention do you pay to the process?  The quality of the tools and the quality of the material used is what will determine your finished product, not what process you use.
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This picture is a rifling button.  To the right side of the picture is the pull rod which extends for another 40" to the right.  The pull rod is used because you actually pull this through the bore.  You can pull in "tension" whereas you couldn't push a small diameter rod through the barrel in "compression" without the rod bending.


   The left side of the picture is the working end of the tool.  Tungsten carbide, because of its hardness, wear resistance and non-galling characteristics, is required.  The tungsten carbide portion is silver soldered to the tool steel pull rod.  You can see the solder line slightly left of center in the picture, just where the grooves begin.  The grooved section is what imparts the rifling.  It is tapered in the front and the back to a small bearing surface in the middle.  This gives a gradual start to the rifling forming, and the small bearing surface keeps the pulling force lower.

   The width of the groove is the width of the rifling.  The width of the segment between the grooves is the width and bottom of the groove.  The button is a negative image of the rifling.  The angle of the grooves is what makes the rifling twist rate.  A shallow angle for slow twist and a steeper angle for a faster twist rate.  Twist rate can be modified, within a small range, by a mechanical set up on the rifling machine that is timed to turn the barrel at a rate that is slightly plus or minus of what the button's rate of twist is.  This is how I made a "gain" twist.  The barrel blank was turned at the same rate of twist that the button would impress, and as the button approached the muzzle, the blank would be turned slightly faster than the button's rate of twist.

   The left end of the tool is an ironing button.  Now this can be done in two separate tools, a rifling button and an ironing button, or it can be combined into one tool, such as this one.  So in one pass the rifling button forms the rifling grooves and the ironing button uniforms the top of the rifling.  
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This is a cutter grinder.  It should be a compound word because it's use is to grind cutters.  Whether from making them entirely or just re-sharpening them, it's done on this machine.  A cutter grinder is what makes making a compound button, such as the above, possible.  Using diamond embedded cutting wheels, the solid carbide blank has 5 tapers cut into it, and then the longitudinal grooves are equally spaced around the circumference.

   At airgun shows,  "I'm making a big bore airgun out of a paintball gun", was an often heard retort.  I have yet to see one of these guns.  In the last couple of years it's changed with people saying that they're making their own rifling buttons.  One guy told me that his buddy made them for $35. each.  I offered to purchase ten of them promptly.  I never heard from that maker.

   When somebody's trying to BS me about them making buttons, it's easy to disarm them.  I ask them: "What grade carbide are you using?"  "How are you truing your wheels?"  "What cutter grinder do you have?"  If they tell me they're doing it in a lathe, with a tool post grinder, I need only ask them how they perform two particular operations.  These operations are difficult to do properly with a tool post grinder.  Most lathes, even when new, are not "tight" enough to prevent backlash, or rigid enough to prevent chatter.  A clever machinist, who could perform the two most difficult operations with a tool post grinder, would also know that he could use a cutter grinder and do it in one tenth the time.  That time alone would pay for a used cutter grinder.
© 2004 Dennis Quackenbush

The next segment will be a rifling cutter head

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