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Author Topic: Rifle Barrel Making (Part 1)  (Read 212 times)

Offline Dutch-Hunter

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Rifle Barrel Making (Part 1)
« on: November 21, 2017, 01:25:34 PM »
RIFLED BARREL MAKING

Of all the components that make up the rifle it is the barrel that seems to hold the greatest mystique and mystery. Anybody with basic knowledge of engineering practice could make the action. Anybody with metal working experience and machinery could make a bolt action.

But the barrel, that is a different matter. How do you drill such a long straight hole to form the bore? How is the rifling put in? How is the shiny finish in the barrel achieved? And above all, what is that special something that differentiates a so-so barrel from a hummer?

Each operation in the making of a rifle barrel requires a special machine tool rarely found outside a barrel shop. That said, there is no real mystery in making good rifle barrels. But it does take care and attention to detail. In this article I will outline the main processes involved in turning a bar of steel into a rifled barrel, indicating where barrel makers differ in their approach.

The United States is the home of the custom barrel makers and there are literally hundreds of small barrels across the country, some still make barrels to the customer’s specifications. There are also some very large barrel makers who make barrels primarily for the trade. The common denominator is that making barrels is all they do.

THE STEEL.


The barrel of any firearm is a pressure vessel with the action serving to stop up the rear end. The peak pressures involved during discharge are enormous, 50,000 pounds per square inch or more, and special steels are required to safely withstand these stresses.

Two classes of steel are currently used in rifle barrels. Most barrels for use on hunting rifles and in military firearms are made from a high alloy Chrome Molybdenum steel of the sort used in high stress components such as axle shafts. In the United States these steels are designated as 4140, 4150 and 4340 types.


In target shooting stainless barrels have for the most part supplanted the use of Chrome Moly barrels. The steel is not a true, fully austenitic stainless such as is found in cutlery. The 416 type stainless steel used in barrels is one of a group of martensitic steels which can be hardened by heat treating like regular Carbon steels. 416 stainless is more accurately described as a "free machining, rust resistant" steel having a high Chrome content, around 10%, but with sulphur added to give it good machining qualities. It is widely considered that stainless barrels will have a longer life and are more accurate than Chrome Moly barrels. If stainless barrels are "shot in" using the prescribed procedure, the barrel acquires a burnishing which almost eliminates fouling, so making stainless barrels very easy to clean.

Because stainless steel is more expensive than Chrome Moly steel and it is more difficult to black due to the Chrome content, high production makers of hunting and military rifles prefer to use Chrome Moly steels. But target shooters who want the best possible accuracy from their barrels are almost without exception choosing "stainless" barrels these days.

The tensile strength of the steel is measured as the force required to break a rod of steel having a one inch cross sectional area by pulling it from its ends. The tensile strengths of steels used for making barrels should exceed 100,000 lb per square inch giving at least a factor of two safety margins over the chamber pressures experienced during firing. But the impact strength of the steel is probably even more important, or the ability of the steel to withstand an impact without breaking. Generally speaking, the tensile strength of a steel alloy can be increased by hardening it. But as the hardness is increased, so the steel becomes more brittle and it becomes more susceptible to fracturing from a hard knock or sharp. A trade off must therefore be made of tensile strength against impact strength and for barrel steel the resultant hardness settled on is usually between 25 and 32 on the Rockwell C scale.

The heat treatment and other production processes involved in making the steel bar leave residual stresses, which can result in the bar bending as steel is removed in making the barrel. The stress can be relieved by putting the steel in an oven and taking it up to 600 C, then allowing it to cool very slowly. Barrel steel is usually double stress relieved to make absolutely sure it stays straight through the subsequent machining processes.

DRILLING

Anybody who has tried to drill a straight hole more than an inch or so deep with an ordinary twist drill will know the problem. No matter how careful you are in lining the drill up to start with the hole will wander and bend and the drill will not come out quite where you expected. So how is it possible to drill a hole several feet down a rifle barrel and for the drill to come out to within a few thousandths of an inch of the center?

The answer of course, is not to use a twist drill. Special drilling machines known as Gun Drills or Deep Hole Drills are used to drill deep accurate holes. On older Gun Drills it is the barrel is rotated at and the stationary drill is fed through it. The drill bit itself is asymmetric, cutting on one side only, and is make of Tungsten Carbide.

The bit has a hole through it and is mounted on a long steel tube with a V groove down its outside. Coolant oil is forced down the tube to cool the drill and clear the filings the drill produces. The drill only cuts on one side and the oil carries the filings up the V groove on the outside of the drill and drill shaft. The drill is designed so that the forces acting on the drill tip tend to keep it on the central axis of rotation of the barrel. The drill progresses down the barrel at about one inch a minute, so it takes about half an hour to drill a barrel. This process and the drills themselves have remained relatively unchanged for the last hundred years except that the drill tips are now made of Carbide.

The finish that the drill leaves inside the hole can be very good, but that is the exception rather than the rule. The hole is usually drilled about 5 thousandths under the size of the bore diameter and then a reamer is used to bring the hole up to size leaving a fine finish and a hole of uniform diameter from end to end.

REAMING

In most instances after the barrel has been drilled, it is reamed up to the bore size. Reaming a good hole is still something of an art. The vast majority of barrel makers that cut rifles or button rifles their barrels will ream prior to rifling. The makers of hammer forged barrels require a very fine surface finish in the bore and they invariably hone their barrels to get the required finish.

The reamer is mounted on the end of a long tube through which the coolant oil is pumped, but at far lower pressures than are used in the Gun Drill. Now it is the reamer that is rotated and the barrel is pulled over the reamer at about one inch a minute.

Over the past few years there has been a quiet revolution in reamer technology and these days most bore reamers are made of Tungsten Carbide instead of High Speed Steel. Reamers made from Carbide last at least ten times longer than HSS ones and generally leave a superior surface finish. Reamer shape has also changed. Reamers have become shorter over the past ten years and do not have pilots on them as reamers of old.

After reaming, the resultant hole has a good finish and has good dimensional uniformity along its length. The barrel is now ready for rifling.

RIFLING

There are currently three main methods by which rifling is put into the barrel:
1.   Cut Rifling
2.   Button Rifling
3.   Hammer Rifling

CUT RIFLING

By far the oldest method, invented in Nuremberg in around 1492, is the cut rifling technique. Cut rifling creates spiral grooves in the barrel by removing steel using some form of cutter. In its traditional form, cut rifling may be described as a single point broaching system using a "hook" cutter. The cutter rests in the cutter box, a hardened steel cylinder made so it will just fit the reamed barrel blank and which also contains the cutter raising mechanism.

The cutter box is mounted on a long steel tube, through which coolant oil is pumped, and which pulls the cutter box through the barrel to cut the groove. As it is pulled through it is also rotated at a predetermined rate to give the necessary rifling twist. A passing cut is made down each groove sequentially and each cut removes only about one ten thousandth of an inch from the groove depth.

After each passing cut the barrel is indexed around so that the next groove is presented for its passing cut. After each index cycle the cutter is raised incrementally to cut a ten thousandth deeper on the next cycle, this process being continued until the desired groove diameter is reached. It takes upwards of an hour to finish rifling a barrel by this method.

BUTTON RIFLING

Up until WW2 rifling was the most time consuming operation in making a rifle barrel and so a lot of effort was put into finding a way to speed up this process. Button rifling is a process that has been flirted with on and off by various large ordinance factories since the end of the 19th century. Today, button rifling is a cold forming process in which a Tungsten Carbide former, which is ground to have the rifling form in high relief upon it, is pulled through the drilled and reamed barrel blank. The lands on the button engrave grooves in the barrel as it is pulled through.

The machinery is quite simple. The button is mounted on a long rod of high tensile steel which is passed through the barrel blank and attached to a large hydraulic ram. The button is mounted in a "rifling head" that rotates the button at the desired pitch or twist as the button is pulled through the barrel. The process takes about a minute to complete.

These are "pull" buttons that are pulled through the barrel. There are slots cut into the button which means 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 “sizing” button follows the “pull” button which presses the burrs back down, so leaving the land tops conforming to the bore circle as they should.

Breaking the pull-rod or pulling the button off the pull rod is a constant danger in "pull" button rifling, so there are several manufacturers who prefer to push the button through the barrel. In this version of the method the button is not attached to the rod, which simply pushes the button up the barrel under the influence of a large hydraulic ram. The trick here is to support the push-rod as it enters the barrel to stop it buckling from the huge forces involved.

There is much opinion that "pull" button rifling is best because the button is kept straight and true as it is pulled through, whereas when pushing the button though the barrel there is an inevitable tendency for the button to tip and yaw so leading to variable bore dimensions. Push-buttoning protagonists deny that this is a problem.

While the process is simple, the technology required to get good results is quite advanced which is why it was not until the middle of this century that it became a generally used technique. It was perfected in the late 1940's at the Remington factory at Ilion largely due to the efforts of Mike Walker, who used the workshop of Clyde Hart in nearby Lafayette for some of the experimental work. The button must be very hard and also tough enough not the break up under the stresses involved as it is pulled through the barrel. The lubricants used to keep the button from getting stuck in the barrel must not break down under the very high pressures involved - it takes around 10,000 pounds of force to pull a button down a barrel. The sort of lubricants used in the press moulding business are what button barrel makers pick through to see what suits, though most makers of button rifled barrels are very secretive about lubricant they use!

Button rifling in its common form is an American development and the overwhelming majority of barrels made in the US are rifled this way. Custom shops such as Hart, Lilja, Shilen and the large high production barrel makers like Douglas and Wilson Arms use the buttoning method to rifle their barrels. The technology has spread and there are a few other small custom barrel makers around the world who do button rifling. Neville Madden (Maddco) and Dennis Tobler in Australia. Anshutz in Germany, better known for their .22 target rifles but also a large producer of hunting rifles also button their barrels.

HAMMER RIFLING

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.

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. Now, Ruger in the US has started making barrels using this method.
 


« Last Edit: November 21, 2017, 03:02:27 PM by Dutch-Hunter »
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Offline MichiganLouie

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Re: Rifle Barrel Making (Part 1)
« Reply #1 on: November 21, 2017, 05:32:13 PM »
Pretty interesting info.  When I worked as a machinist, I turned a lot of 410, 416, and 4140.  416 was the nicest to work with.  Also machined 302, 304, and 316, and CRS.
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