THE MAKING OF A RIFLED BARREL.
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
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 aquires 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 margin over the chamber pressures experienced during firing. But the impact strength of the steel is probably even more important, this being the ability of the steel to withstand a sharp knock without breaking. Generally speaking, the tensile strength of a steel 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 impact - or setting off a small explosion inside a tube of the stuff! 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 over the next twelve hours or so. Barrel steel is usually double stress relieved to make absolutely sure it stays straight through the various machining processes.
DRILLING THE HOLE.
ever you make in life, you have to start with a hole.” Ernie Stallman, Badger
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 centre?
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 most older Gun Drills it is the barrel itself which is rotated at speeds of between two and five thousand r.p.m. and the stationary drill is fed through a tight fitting guide bush into the end of the barrel. 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 at 1000 p.s.i or more to cool the drill and clear the swarf the drill produces. The oil and the swarf come down the V groove on the outside of the tube, the oil is then strained to remove the swarf before returning to the main tank. 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 unchanged for the last hundred years - except that the drill tips were not made of Carbide back then!
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.
TIP OF A GUNDRILL.
Doesn’t look much like a regular twist drill!. Cutting oil is pumped at high pressure through the holes at the end of the drill. The drill only cuts on one side and the oil carries the swarf 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.
DEEP HOLE DRILLING MACHINE.
The gundrill is attached to the pressure head at the bottom of the photo, where cutting oil is pumped down the drill shaft at high pressure. The drill is supported in the middle by a steady to give it added rigidity. The drill passes through a series of guide bushes before entering the barrel, which is being rotated by the spindle-head at the top of the photo. Oil is seen coming off the spout attached to the guide-bush-head and carries the swarf out from the hole in the barrel. The oil returns the main tank after passing through strainers to remove the swarf.
REAMING THE HOLE.
“When two or three barrel makers gather together, the conversation turns to the difficulties and problems of reaming a good hole.” Observations of a barrel maker.
Every barrel maker I can think of who cut rifles or button rifles their barrels will ream prior to rifling. 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, at about 200
rpm and the barrel is pulled over the reamer at about one inch a minute.
Reamers made from Carbide last at least ten times longer than HSS ones and generally leave a superior surface finish. They can also be run at much higher feeds and speeds - 500 R.P.M and 10 inches a minute is not uncommon! Reamer shape has also changed. Reamers have become shorter and shorter over the past ten years and do not have pilots on them as reamers of old.
Reaming a good hole is still something
of an art though. Several barrel makers I know refuse to buy bore reamers
claiming that you cannot buy a good bore reamer and I have to say there is
something to that. Barrel makers who do buy their bore reamers get them from
the reamer makers who advertise in this magazine, (Precision Shooting), but
generally the reamer needs some hand honing to get it to “run right” and
leave a good finish. In my experience, the only reamer maker whose reamers do
not require attention before using them is Dan Green of Forgreens.
Dan is a really great reamer maker and his chamber reamers are also quite
outstanding - I only wish he would make reamers in Carbide!
BORE REAMER ENTERING THE BARREL.
After the barrel has been drilled, it is reamed up to the bore size. Here a bore reamer enters the barrel as copious amounts of cutting oil flushes the chips away.
There are currently three main methods
by which rifling is put into the barrel. By far the oldest method, invented
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.
The rifling machines found in custom barrel shops are invariably Pratt & Whitney machines. For the first world war some thousands of “Sine Bar” riflers, so called because a sine bar is used to determine the rate of twist, were built to satisfy the demand for barrels at that time. These belt driven single spindle machines weighed about a ton and were suitable for the wooden floored workshops of that era. After WW1 many of these machines became available quite cheaply on the surplus market and so in the inter-war years these were the standard rifling machine in barrel shops across the World.
The classic rifling machine! P & W started making these machines over 100 years ago and these “Universal” rifling machines formed the backbone of the barrel making business up until WW2. This one was made in 1895!
At the start of World War Two, Pratt & Whitney developed a new, “B” series of hydraulically powered rifling machines, which were in fact two machines on the same bed. They weighed in at three tons and required the concrete floors now generally seen in workshops by this time. About two thousand were built to satisfy the new demand for rifle barrels, but many were broken up after the war or sold to emerging third world countries building up their own arms industry.
Very few of these hydraulic machines subsequently became available on the surplus market and now it is these machines which are sought after and used by barrel makers like John Krieger and “Boots” Obermeyer. In fact, there are probably less of be “B” series hydraulic riflers around today than of the older “Sine Bar” universal riflers.
In the late 1930’s the venerable Sine Bar rifler was replaced by the “B” series hydraulic rifling machines. These twin spindle machines were altogether more massive, more rigid and more powerful. During WW2, much faster methods of rifling were developed and superseded single point cut rifling as a technology for mass production of rifled barrels. These machines remain, then, the zenith of cut rifling technology.
During World War Two several other methods of rifling barrels were developed which greatly speeded up and simplified the process. So the Pratt & Whitney “B” series of Hydraulic riflers remain the last word in cut rifling machine technology.
Due to the very limited availability of these machines there are several barrel makers who have made their own machines. But, as will be appreciated from the description of the process above, these machines are complex and expensive to build.
The techniques of
cut rifling has not stood still since the end of the war though.
Largely due to the efforts of Boots Obermeyer the
design, manufacture and maintenance of the hook cutter and the cutter box has
been refined and developed so that barrels of superb accuracy have come from
his shop. Cut rifled barrel makers like John Krieger (Krieger Barrels), Mark Chanlyn (Rocky Mountain Rifle Works) and Cliff Labounty (Labounty Precision Reboring) who are fast growing in prominence for the
quality and accuracy of their barrels, learned much of their art from Boots Obermeyer, as did I.
Grunig & Elminger
The heart of the cut rifling method, making and maintaining these cutters require great skill
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.
“Any fool can pull a
button through a barrel!”
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. 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.
The barrel can be seen in the middle of the picture. The right hand end of the barrel is held against a thick steel pressure plate. The button is seen about to enter the barrel at its left hand end. At the other end of the barrel the pull-rod is attached via the twisting-spindle to a frame, which is pushed by the two hydraulic rams above and below the barrel. As the frame is pushed to the right the button is pulled through the barrel - it needs a lot of force to pull a button through a barrel! The twisting-spindle is driven by the rack and gear seen at the right hand end of the photo so that the button is positively driven at the desired rifling twist, preventing button slippage.
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 like Hart, for example, 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 however - as of course, they would!
Whilst 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
Button rifling in its common form is
an American development and the overwhelming majority of barrels made in the
The technique of hammer forging rifle
barrels was developed by
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
Hammered barrels have never achieved much favour 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
Profiling the barrel can be done on a regular lathe, but as the barrel is relatively very thin for its length it is not very stiff it is difficult to machine the middle part of the barrel without inducing a lot of chatter, which can ruin the finish and in bad cases even bend the barrel. Also, one is really limited to straight taper profiles achieved by offsetting the tailstock. To reproduce the curving lines of most sporter and bigbore target rifle barrels in a reasonable time requires the use of a proper profiling lathe which has a hydraulic copy attachment and a self centering steady.
The hydraulic copy unit has a sensitive stylus which follows the shape of the pattern, which is usually mounted on a rail behind the lathe bed. As the automatic feed moves the saddle down the bed of the lathe, so the stylus follows the contours of the pattern. The cutting tool is mounted on a hydraulically actuated tool post and mimics the movements of the stylus, so reproducing the shape of the pattern.
To hold the barrel steady and stop it from vibrating a hydraulic or pneumatically operated steady follows a few inches behind the cutter. This consists of three rollers which clamp on the barrel and which are linked so that if one moves radially in or out then the others follow it. This allows the steady to adjust for the changing diameter of the barrel as the tool and steady move from the thin muzzle the thick reinforce.
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. 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 at the muzzle.
The cutting tool is attached to a
hydraulic copy unit, the pattern following stylus of which can be seen in the
bottom right hand part of the picture near the pattern barrel. The three jaw self centering steady can be seen in the middle of the
picture. This follows just behind the cutting tool, keeping the barrel rigid.
“No need for it, just wears the barrel
Lead lapping the barrel is done to polish the bore and remove machining marks and also to remove any tight spots in the barrel and make it dimensionally uniform end to end. If you were to sit down and write a top ten list of barrel makers, past or present, almost all would lap their barrels.
This process is usually done by hand,
though the process is mechanised in larger shops.
It also acts an inspection process for the barrel maker who can feel what is
going on up the barrel.
Lapping the barrel adds between one and three tenths of a thou’ to the bore and groove diameters of the barrel and is used by most small custom barrel makers as the finishing process on the inside of the barrel. Lapping a barrel will improve the performance of almost any barrel - in some cases, startlingly so!
Generally, lapped barrels will shoot well from the word go where as the same barrel not lapped may take a thousand rounds or so until it starts performing at its best. Contrary to Ackley’s dictum, lapping will add to the accurate life of a barrel, not detract from it.
Some people want their barrel
dimensions accurate to the nearest tenth of a thou’.
But in truth, the golden rule seems to be that the groove diameter must be
same or less than the bullet diameter to get good accuracy. It does not seem
to matter how much less you make it, one tenth of a thou’
or one thou’ - so long as it is less.
Some people say that the finish in the barrel is important and the better the finish, the better the barrel. What seems to be true is that if the finish is too good, less than 10 microinch roughness, then metal fouling tends to become a problem. It seems that in a “rough” barrel, the bullet rides on the high spots and so there is relatively little friction. But the smoother the barrel gets then the greater the surface contact with the bullet. The friction goes up and more bullet jacket gets left behind. Of course, if the barrel is too rough, then this just picks up metal and acts like sandpaper on the bullet. But there is a band of surface roughness where metal fouling is minimised which is between 10 and 20 microinches.
There have been many claims over the years that different forms of rifling profile will give better results. But so far, there is no conclusive proof that the so called concentric form almost universally used these days is any worse than any other - or any better!
What is important is that the bore and groove dimensions are uniform down the length of the barrel, that the twist rate is uniform and that the groove circle is concentric with the bore. Many think a slight choke at the muzzle end of a ten thousandth or so will improve accuracy. But bench rest shooters have shown that parallel barrels seem to win more matches than choked ones. The exception is barrels which shoot lead bullets, like .22 Rimfire barrels and air rifle barrels. These barrels definitely shoot better if there is a slight choking in the barrel. The barrel should also be completely free of stress so that as it warms up during a course of fire the barrel does not bend so leading to group shifting.
WHICH METHOD MAKES THE BEST RIFLE BARRELS?
Here that at Border Barrels we used to exclusively cut rifled until the demand for our barrels outstripped capacity. The principle problem was (and is) the availability of cut rifling machines - you don’t exactly trip over them at machinery auctions. Also, a high level of skill is required to maintain the tooling. Like all old technologies, cut rifling is slow and requires a high level of skill to make and maintain the tooling. This makes it expensive. The other side of the coin is that the tooling is readily made with simple machine tools and is very flexible - by which I mean that a large range of twists and bore dimensions and numbers of grooves are readily achieved using the same cutter and cutter box. This makes cut rifling is an efficient way to go if you are making custom barrels in one’s and two’s to a customers specific requirements and you can charge a hefty sum for your efforts. But cut rifling machines are expensive to buy and expensive to operate.
Having started out as a custom barrel
maker making custom barrels in one’s and two’s, we found that more and more
of our work was in longer and longer runs of barrels of the same type and
when it takes an hour or more to rifle a barrel, it sure leads to long days.
If it is so easy to rifle barrels using a button, why do some barrel makers persist in the difficult, time consuming art of cut rifling? As outlined above it is critical in an accurate barrel that bore and groove dimensions be uniform end to end. When buttoning a barrel then it is critical that the steel be very homogeneous and of uniform hardness along the bar. If not then the button will engrave deeper into the soft parts than the hard parts so giving varying dimensions down the barrel. The button rifled barrel maker is very much in the hands of his steel mill to supply him with uniform, homogeneous steel.
The depth of engraving of the button
depends also on the thickness of the bar at the point where the button is. If
the bar is thin then the metal can then just expand elastically allowing the
button the pass without doing much engraving. If the bar is thick there is
more resistance to the button and so it will engrave deeper. To hope to get
uniform bore dimensions it is critical to pull the button through a blank
which has the form of a parallel cylinder. A button rifled barrel must be
rifled and then stress relieved before it is profiled. There is always the
problem that any residual stresses are going to make the barrel move when
profiling, so leaving you with a barrel that is bent or bell-mouthed.
Cut rifling a barrel puts no stress in the steel and so it is possible to profile the barrel after the drilling stage. Any moving around the barrel is going to do will be done and the barrel can be reamed and rifled after profiling. The problem of stress induced changes in bore dimensions during manufacture can be eliminated.
When making fluted barrels, the fear is always there that putting, say, six flutes on the outside of the barrel will leave the bore shaped like a hexagon instead of round. (Like it should be!) With cut rifling, the barrel can be completely profiled and fluted after the drilling stage and then reamed and rifled. All the niggling doubts as to whether fluting the barrel will ruin it can be dispelled if it is done this way - but this manufacturing route in not available to you when you button rifle a barrel.
The exact twist of a buttoned barrel is also unpredictable. The button tends to slip in the barrel so what set out to be a 12 inch twist may end up as a 12.5 inch twist. This is not a problem if the twist is uniform, but if it varies down the barrel - particularly if it slows - then like as not, it will not shoot.
Another problem, is that the button may not engrave as deep on one side as on the other so leaving a groove circle that is not concentric with the bore. The result is bullets leaving the barrel which are not balanced and so unstable. This becomes less of a problem if you have lots of shallow grooves instead of a few deep ones.
Selection is the key to success with buttoned barrels and barrel makers like Hart, Douglas and Shillen grade their barrels by using an air gauge to judge the uniformity of bore and groove dimensions in each barrel. Ultra Premium Select barrels carry a premium price tag and are used by the bench rest fraternity. The lower grades get turned into regular target barrels and sporter barrels.
None of these problems arise in traditional single point cut rifling a barrel. Groove circle and bore are always very concentric. Because very little work is being done on each passing cut the twist rate is very consistent and very uniform. As a general rule, I find that button rifled barrels are not as uniform in bore dimensions straight off the machine as a cut rifled barrel. Button rifled barrels usually need a deal more lapping than a cut rifled barrel due to this problem.
Exact dimensions are easier to achieve by cut rifling and are not dependent on the hardness or thickness or type of steel as they are in buttoning. No stress is put into the barrel by cut rifling so no stress relieving is needed.
Many people think that the superior
finish in a button rifled barrel must mean it will shoot better. In the first
place, the surface finish that counts is the longitudinal surface finish,
down the length of the grooves. The tool marks in cut rifled barrels go in
just this direction, so causing mini-lands which are quite uniform down the
length of the barrel. The transverse “roughness” caused by the lands are
never viewed as an impediment to accuracy, so the mini-lands left by the tool
marks in cut rifled barrels will also not affect accuracy. In the second
place, the final finish in the barrel is determined by the lapping. If the
cut rifling barrel maker has paid attention and kept his cutter sharp to
minimize tool marks, then after lapping only a very experienced eye will be
able to tell the difference between the cut rifled and the button rifled