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IHMSA News Feature Article
Published in The IHMSA News, the Official Publication of The International Handgun Metallic Silhouette Association
Published monthly except November/December - January/February
IHMSA on the web at http://www.ihmsa.org
 
The Art and Science of Barrel Making
By Todd Spotti
 
     Barrels. Every gun has at least one. Everyone talks about them, but at the same time, very few shooters really know anything about how theyíre made. Have you ever wondered how the barrel maker turns a rather ordinary looking steel rod into something that can deliver bullets on to the smallest of targets at incredible distances time after time?

"No matter the type of barrel, careful
attention to detail is critical to produce a quality product."

     Itís a fantastically complex and demanding task, and the barrel makerís attention to fine detail is extremely important to good future accuracy. You can have the most highly engineered action, the most expensive stock, the most sensitive of triggers, and a well proven load, but if theyíre matched up with a so-so barrel, youíve got problems. All that aside, the more you know about barrel making, the better the questions you can ask when shopping for your next re-barrel job. Letís take a look at how it all gets done.

The Steel

     The steel for rifle barrels comes from various small specialty mills and is often delivered in 20 ton lots, which is a tiny amount by regular steel mill standards. The delivered bars can run anywhere from 12 to 20 feet in length and are usually 1.25" in diameter. Additionally, they may or may not be stress relieved at the mill and their chemical composition and quality may or may not be certified (depending on whether the barrel maker wants to pay for it). However, it is extremely important that only the highest quality steel be used for barrel making. That means that there are absolutely no voids in the material, no carbide crystals, nodules, or other impurities embedded within, no soft or hard spots, and the steel is perfectly homogeneous throughout.

     Barrel steel should have three basic characteristics i.e. machinability (the ability to be easily cut and shaped), durability, and strength. The two kinds of steel that are used most often for rifle barrels meet these demanding criteria. One is 4140 chrome moly steel which is a type that is most commonly used in high stress applications such as in truck axles and various bulldozer parts, etc. The other is 416 stainless. This type of steel is not your ordinary stainless variety. While it contains 10% chrome for corrosion resistance, it also contains sulfur to improve its machinability. Like chrome moly steel, itís also capable of being heat treated to increase its hardness to make it even more strong and durable.

     Obviously you can overdo the hardness to the point that the steel can become brittle, so typically most barrels, no matter what the material, will run between 25-32 on the Rockwell scale. This means that their strength will be able to handle pressures of at least 100,000 psi. If you need a barrel that can handle an even larger amount of pressure, you should change your load.

     After delivery to the barrel maker, the bars will then be cut to length (around 28-30") and then trued up so that theyíre perfectly round and the ends will be faced off. Having a perfectly round bar and perfectly parallel sides is extremely important as youíll soon see. Then, if the bars have not been heat treated at the mill, theyíll be heat treated to remove the stresses induced into the barrels from the steel making process. This usually involves baking the bars in ovens at around 600 degrees centigrade with a slow, controlled cool down taking anywhere from 12-24 hours depending on the chemical composition of the steel.

Drilling

     This is definitely the most difficult part of the whole process. Ever try drilling a long, straight hole? Itís not easy. It always seems like the drill bit never comes out in exactly the place that you intended. Here, weíre talking about drilling a relatively small hole for thirty continuous inches in some very hard material. However, barrel makers have come up with a very unique approach to this problem.

     They mount the barrel in a fixture and then spin the barrel at anywhere from 2-6 thousand rpm. The drill bit is stationary and is mounted on a 40 inch rod that is then moved into the spinning barrel at a rate of around an inch per minute. As you can see this is a slow process.

     The bit is made from tungsten carbide steel and is shaped with a cutting lip on only one side of the tool. I know it seems strange, but thereís a reason for it. Both the drill rod and bit are hollow and cutting oil is forced through them at anywhere from 1,000-1600 psi to flush out the resulting chips/swarf. Because the bitís cutting surface is on one side only, room is left on the opposite side for the metal chips and oil to flow back out of the hole being drilled.

     You can see why having the barrel perfectly trued and round is vitally important if itís going to be spun up at these very high speeds. If it weren't, itíd be wobbling, or perhaps even flying all over the place. Even a very slight imbalance would have serious consequences as it would cause the barrel to bow as it was being spun. That means while you could drill a straight hole in a bowed spinning barrel, when you stopped the spinning, the barrel would then straighten out but now the hole would be bowed.

     Another problem the barrel maker has to confront is making sure the drill bit, which is soldered on to the drill rod, is perfectly straight and true. If itís not, the hole wonít be straight. Additionally, they have to make sure the bit is perfectly centered on the barrelís center and that the bit wonít wander, flex, or chatter as itís being moved on that long 40" rod into the barrel. Lastly, your machinery has to be in tip top condition. Any wear on the high speed precision bearings of your gear will translate into an inaccurately drilled barrel. As they say, the details will get you every time. Barrels are usually drilled around .005" under the desired bore size, which brings us to reaming.

Reaming

     Even though the barrel maker has taken great care in drilling process, the surface finish of the hole wonít be as smooth as we would wish. This is where reaming comes in. The reaming process will bring our barrel hole up to final bore dimensions and will provide us with the final finish of the surface (unless we have the barrel lapped).

     In this case, the barrel is now stationary and itís the reamer that is rotated. The reamer is located on the end of a long shaft which is then spun at 500 rpm. The barrel is now moved forward onto the spinning reamer. Both the reamer and the shaft itís mounted on are hollow, and as before, cutting oil is pumped through at 200 psi to cool, lubricate, and flush out the swarf.

     As before, itís critical that the reamer and the barrel are perfectly aligned and that the reamer and its rod wonít flex, wander, or chatter. If this operation is done properly, weíll have a very smooth hole. If not, weíll have horizontal marks around the diameter of the bore. In my examination of many, many barrels over the years with and without a bore scope, this is the most common barrel flaw that Iíve found. Custom barrel makers do an excellent job in reaming. Barrels on mass manufactured firearms often will have reamer mark on tops of the lands and sometimes even in the grooves. I would guess that poor maintenance of the reaming tools is probably the biggest culprit for this situation. Now itís time to rifle the barrel.

Cut Rifling

     Cut rifling is the oldest of the various types. It was invented in Germany in 1492 - a very good year for more than one reason. As the name implies, a cutting, or maybe a better word might be a shaving tool, scrapes away the metal in a spiral pattern to form the grooves of the barrel.  A cutter hook is mounted in a "cutter box" or carrier which fits inside the bore. Like a wood plane, the cutter box is pulled through the barrel shaving away .0001" of metal. After itís pulled all the way through, the barrel is rotated and the cutter box starts shaving the next groove. After all six grooves are done for the first time, the cutting hook is adjusted another .0001" lower and a second, third, etc. pass is made until the desired depth of the grooves is achieved.

     As you might guess, this is a slow process, and it takes somewhere around an hour to rifle a singe barrel. It also takes a very skilled operator to run and adjust the machinery. Consequently, cut rifled barrels are more expensive to produce than the other types. However, on the plus side, the barrel incurs absolutely no metal stresses what so ever. Indeed, cut rifle barrels like those from Krieger, have produced remarkable results on the benchrest circuit.

Button Rifling

     Button rifling was perfected during World War II when it became clear that the slower cut rifling methods of the day couldnít satisfy the tremendous demand for barrels. The process was perfected by Remington who used the nearby facilities at Hart Barrels for its experiments during the development.

     In this process, which is the most common in the U.S., a carbide "button" is passed through the bore to form the rifling. First the bore has to be lubricated. Every barrel maker has their own "secret sauce" or lube and guards its identity fiercely. A button is a somewhat football shaped carbide tool with the rifling pattern ground in relief into its surface. The button is attached to a rod and is then pulled through the bore. (Hart pushes them through instead.) As the hard button passes through, the raised rifling pattern on its surface is pressing into the softer surface of the bore and is creating the grooves in a cold forming process.

     The operation is very fast, and only takes about a minute per barrel. Thus, button rifled barrels are usually less costly to produce. There are two types of buttons. One is a simple rifling button which works as just described. On the minus side, a simple button will leave burr like feathers on the edge of the lands. However, a combo unit which consists of a rifling button and a finishing button will both press in the lands and smooth their edges in the same pass.

     As you might guess, pulling an oversized button through an undersized hole requires great force and creates significant stress in the barrel. As a result, itís mandatory that button rifled barrels be stress relieved after the process. If not, all kinds of strange things including splitting down the length of the barrel is possible.

Hammer Forging

     Again, it was the demands of war that was responsible for the development of this process - only it was done by the other side. Hammer forging was developed in Germany in 1939. Here a drilled barrel, rather than being reamed, is honed to give it a very fine interior finish. Then it is placed on a tungsten carbide mandrel that has the entire rifling pattern ground in relief into its surface. The barrel/mandrel combo is then placed between two opposing power hammers and rotated. The hammers literally beat the barrel into the mandrelís pattern. Iím told a barrel will actually grow around a third of its length during this process. It usually takes around three minutes for the rifling process to be completed. As you would think, this method produces tremendous stresses in the barrel that have to be relieved through heat treating.

     The advantages of hammer forging is the fact that the interior finish is very good, and the bore surface becomes work hardened in the beating process. The result is a very durable, long lasting barrel. Modern hammer forging has progressed to the point that even the chamber can be included in the mandrel pattern. These machines are very large, complex, and expensive however. So the small custom barrel shops are pretty much eliminated from using them.

     Additionally, some say that the induced stresses are so severe in this process that they never can be entirely eliminated. As a result, the bench rest crowd wonít touch a hammer forged barrel. However, I canít help but wonder how valid this belief actually is, and whether anyone has actually tried it. Remington barrels, which always had a reputation for accuracy, used to be hammer forged, including those used in the XP-100. I know I always had excellent results with original XP barrels. Indeed, in Europe, hammer forging is the standard. Sako, Tika, H&K, Steyer, and Sauer all use hammer forged barrels. Iíd hardly call their products junk.

Lapping

     Most premium barrels are lapped. The unanimous verdict of just about every expert in the barrel making arena is that lapping does indeed improve accuracy. Lapping accomplishes this by polishing the interior surface smoother and eliminates any tight spots in the bore.

  Lapping is generally a hand operation. A rod with a handle at one end is inserted into the new barrel. Molten lead is then poured down the muzzle around the rod for a distance of around 4". The lead then hardens and the lead plug or lap is tapped out. The lap now has the perfect relief pattern of the interior of the bore. Lapping oil or lapping compound is placed on the plug which is then passed back and forth in the polishing process. An experienced person can readily feel any tight spots and then work to eliminate them to produce a near dimensionally perfect bore. Again, all this hand work costs money, so only the very top quality barrels will be hand lapped.

     Now you might be thinking, "Well I can improve the accuracy of my unlimited gun by lapping the barrel my self". Donít do it. Lapping will increase bore size. The barrel maker has already taken this into account when ordering his buttons so when he laps, the final diameter will meet specification.

     Lapping a mounted barrel will especially increase the bore diameter at both ends of the barrel. This isnít a problem for a new barrel since one end is going to be crowned and the other chambered. However, on an existing mounted barrel which already has a chamber and crown, throat diameter will be enlarged and so will the muzzle. This is especially critical at the muzzle because itís very possible that the bullet wonít be fully supported when it exits and will tip and yaw excessively as a result. Accuracy is likely to be very poor.

     Let me end this piece by making a comment about breaking in a new barrel. Iíve written about this in the past so I wonít repeat the details of the process and will just say that if you want the very best accuracy that your barrel can deliver, you should take the time to follow a break in procedure. Why? Because even the very best barrels will have small imperfections on top of the lands and in the grooves. When you follow a break in procedure, youíre giving your barrel a final polish that will smooth out these imperfections, and your new barrel will be shooting its best in the least amount of time. Accuracy reducing fouling will be also minimized. I know itís a pain, but itís a good investment in future accuracy.

     So there you have it. If you keep this information in mind when shopping for an aftermarket barrel, youíll be more likely to make the best choice for your needs. A good barrel is like a good dog. They give so much pleasure that youíll remember them long after theyíre gone.

Good luck and good shooting, Todd

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Warning: All technical data mentioned, especially handloading, reflect the limited experience of individuals using specific tools, products, equipment and components under specific conditions and circumstances not necessarily reported in the article or on this web site and over which IHMSA, The Los Angeles Silhouette Club (LASC), this web site or the author has no control. The above has no control over the condition of your firearms or your methods, components, tools, techniques or circumstances and disclaims all and any responsibility for any person using any data mentioned. Always consult recognized reloading manuals.