VARIOUS METHODS OF MAKING A RIFLE BARREL

Three types of rifling
There are three principal ways rifled barrels are made today, and two of them start with a long tube of metal. They are the cut-rifled barrel and the button-swaged barrel. The other - hammer forging - is quite different.

The following process refers to both cut-rifled barrels and button-swaged barrels.

To get the long tube of metal needed for the barrel, the maker starts with a tube or a solid rod. If they start with a solid rod, the hole through the center must first be drilled. The task of drilling a deep (long) hole through a solid rod of metal is one of the toughest machining tasks known.

Most barrel makers drill this hole on a lathe or special built deep hole drilling machines. The drill bit itself has a special cutting surface to reduce the tendency for the bit to wander. Even so, no hole is ever drilled entirely true. The barrel maker has to use other means to true up the hole if he wants a quality barrel.

The hole is reamed and (possibly) lapped
The next step is to ream the hole. A trueness of about 0.001" along the axis is possible with very careful work. If the process is speeded up or the reamer is dull, it will be 0.0015" or even 0.002" of variation along the entire axis. If the maker is a good one, the next step is to lap the bore.

Lapping does not increase the dimension of the bore. It's purpose is to remove the tooling marks left by the reamer, just as the reamer also removed the larger marks left by the drill bit. The finest lapping is done with a lead slug that is cast right in the bore of the gun, so the fit is perfect. The cooled slug is broken free and pushed halfway out the bore, where fine abrasive power and oil are brushed on. This is called charging the lap.

The charged lap is run up and down the bore, recharging as required to keep polishing the bore. Because the lap is lead, the lapping powder sinks into it before it scratches the steel bore, so this is a laborious process.

Cut rifling
After lapping, the barrel is cleaned. If it's going to have cut rifling, it is now installed in a rifling jig or machine, which looks something like a lathe. A headstock holds one end of the barrel, which is held at the other end so that it can be turned easily. A rifling cutter is a very small tool that fits on the end of a rod long enough to pass completely through the barrel. The cutting rod is mounted on a fixture that causes the rod to spiral as it passes through the bore. Two hundred years ago, this fixture was a wooden positive of the rifling pattern desired. It had the same twist rate that was desired in the rifle. Today, a precision rotating fixture is used.

RIFLING "HOOK" CUTTER. 
The heart of the cut rifling method, making and
maintaining these cutters require great skill.

When a cut is complete, the headstock is indexed for the next groove and another cut is made. When all the grooves have had one pass of the cutter, it is adjusted to cut deeper and another set of cuts is made. Each pass of the cutter will remove about one ten-thousandth of an inch, if the barrel maker is a good one, so each thousandth of an inch takes ten full passes. If the rifling is 0.005" deep, each groove took 50 passes of the cutter. If the barrel has six grooves, it took 300 passes of the cutter to completely rifle that bore. To speed things up, the cutter can be set to cut deeper, but that means more chances for burrs, gouges and associated tool marks.

RIFLING CUTTER HEAD The "hook" cutter is seen sitting in its box in the middle of the cutter head. The cutter sits on a wedge and as the screw at the end of the head is turned, it forces the wedge under the cutter so raising it to increase the depth of cut.

Button rifling - faster and cheaper
Another way to rifle barrels is the most popular method used today. The button rifling method doesn't cut the rifling pattern - it irons it into the barrel! A carbide tool called a button is either pushed (most common) or pulled through the barrel. As it moves, the hydraulic head also turns at the desired rifling twist rate. It is correct to say the button engraves a reverse of itself inside the bore, but it doesn't do it by cutting. Instead, it displaces metal, actually hardening and smoothing the inside of the barrel as it goes.

RIFLING BUTTONS.These are "pull" buttons that are pulled through the barrel. The left hand button is a simple rifling button. The slots cut into the button mean the button does not engrave the barrel in that area and as a result, lands are left in the barrel. The lands left by this simple rifling button tend to have raised burrs on their edges. The combination button on the right has a sizing button to follow the rifling button which presses the burrs back down, so leaving the land tops conforming to the bore circle as they should.

Metal stresses
Unlike the cut-rifling method, the button method sets up stresses in the barrel. These have to be dealt with, or the barrel will "walk" as it heats up during firing. You can always tell a cheap barrel; as it heats up, it starts spraying shots in an ever-expanding pattern.

When profiling a barrel a lot of metal is removed and if there is any stress in the metal then this is relieved by the removal of material. This may result in a barrel that started out as straight ending up as bent. This is not usually a problem when cut rifling a barrel as this does not induce any stress, but button rifling induces a fair amount of radial stress which is relieved by turning the barrel down. What happens then is that as you remove metal from the outside so the dimensions on the inside grow larger.  

For example; If you turn a sporter barrel with a skinny muzzle from a buttoned blank then you find the barrel is bell mouthed and the bore diameter is a thou' or more bigger at the muzzle than the chamber - definitely, not good!

Buttoned barrel blanks have to be stress relieved before profiling to prevent this expansion. In the carbon wrapping process, the steel portion of the barrel is much like a light sporter barrel. It has to be turned and contoured before it is carbon wrapped. In our opinion, stress relieving barrels is a black art rather then a science, and the results vary greatly from one manufacturer to the next as well as one piece of steel to the next.

This is Bettin Custom’s primary reason for using only cut rifled barrels for our carbon wrapping and on our custom rifles.

 While it may take an hour to cut-rifle a barrel with a modern barrel making machine, a button-rifled barrel can be rifled in about a minute. More time will then be spent on relieving stress (or not), but the total doesn't add up to the man hour that's invested in a cut-rifled barrel. And, remember, if the man running the machine makes $20/hour, the barrel will have to include $100 just to cover his cost. The cost of the blank, of drilling the hole and of reaming and lapping are all extra. A good cut-rifled barrel will retail for $300 and up, while quality button-rifled barrels may start at $150.

HAMMER FORGING

The technique of hammer forging rifle barrels was developed by Germany before WW2 because the MG42 machine gun, with 1200 rounds per minute rate of fire, positively ate barrels. The first hammer rifling machine was built in Erfurt in 1939. At the end of the war it was shipped down to Austria ahead of the advancing Russian army, where American technicians were able to get a good look at it.

In this process the barrel blank is usually somewhat shorter than the finished barrel. It is drilled and honed to a diameter large enough to allow a Tungsten Carbide mandrel, which has the rifling in high relief on it, to pass down the blank. The blank is then progressively hammered around the mandrel by opposing hammers using a process called rotary forging. The hammered blank is squeezed off the mandrel like tooth paste and finishes up 30% or so longer than it started.

Today, barrel hammering machines are built by Gesellschaft Fur Fertigungstechnik und Maschinenbau (GFM) in Steyr, Austria. They cost about a half a million dollars and can spit out a barrel every three minutes. These machines have reached a very high degree of development and are so sophisticated that they will not only hammer the rifling into the barrel, but it is also possible to chamber it and profile the outside of the barrel all in the one operation. Only large scale arms manufacturers and ordinance factories have pockets deep enough and barrel requirements insatiable enough that they can afford to buy and run such a machine.

Hammered barrels have never achieved much favor in target shooting. Whilst their proponents laud the virtues of the mirror finish of the bore and its work hardened surface, which gives long life, the barrels tend to be very variable in the uniformity of their dimensions down their length. Also, because the metal is worked completely throughout the barrel there are considerable radial stresses induced which are difficult to remove completely by the usual stress relieving methods. Stainless steels tend to work harden to a much higher degree than Chrome Molybdenum steels and so do not remain malleable enough to hammer forge. Because of this, it is difficult to make stainless barrels this way. Stainless barrels are being hammer forged, but using type 410 steel which has a lower chrome content than the regular 416 steel usually used for making barrels by other methods.

The following barrel makers use hammer forged barrels on their rifles.

Most of the big hunting rifle makers in Europe hammer forge their barrels. Sako and Tikka in Finland, Heckler & Koch, Steyr and Sauer in Austria.

 In the U.S, Ruger, Remington, Winchester, FN, Weatherby, Browning, just to name a few, make barrels using this method.

Twist rates and groove dimensions

With the cut rifling technique, the twist rate can be infinitely variable. The same cutter can be used for a variety of twists, so the barrel maker only has to change the setting of the rifling machine. In contrast, the button used for button rifling, has the twist rate pre-determined when the button is made. It is very expensive to make new buttons, so most buttoned barrel makers have a limited number of twist rates available for any given caliber. The same situation goes for the hammer forged barrels as well.

Many cut rifle barrel makers, still use rifling machines built in the 1940’s, these machines wear with time, and are completely mechanically driven with complex gear drives, pulleys etc.

In the case of Satern Custom Machine, their cut rifling machines are custom built CNC machines, capable of holding extremely tight tolerances. They can produce any twist rate desired, down to the 4^th decimal place if desired, by simply programming in the desired twist rate. For example, a common twist rate for a 30 cal barrel is 1:12. We have found that certain bullets like very specific twist rates. We order many of our 30 cal barrels with a 1:11.2500 twist.

Another disadvantage of button vs cut rifled barrels is bore/groove dimensions. A buttoned barrel will be the size of the button, if a different groove dimension is desired, a new button must be built. I the case of a cut rifled barrel, the groove dimension can be made to any depth desired by simply stopping the cutting procedure at the requested depth. As an example, we have found that certain Barnes bullets produce better accuracy if the groove dimension is slightly undersized. We can order our 30 caliber barrels with a groove diameter of .3077” rather then the standard .308” diameter of most 30 cal barrels.

Conclusions

Hopefully this article will help people understand the differences and advantages of the most common rifle barrel making techniques available on the market today. We hope it also explains why we prefer cut rifled barrels for our particular application. Button barrels as well as hammer forged barrels have their place in the industry, as long as every one understands the advantages and disadvantages of the varying production techniques.