Preface
Many shooters don’t realize that barrels have
always been hard to obtain, even in peacetime, due to a national and
worldwide shortage of barrel-making capacity. Prior to 1990, Sturm,
Ruger & Company purchased all rifle barrels used on their
firearms. When it became impossible to purchase enough high quality
barrels, Mr. William B. Ruger, Senior decided his company would make
their own barrels, and he purchased a hammer forging machine. While
the following technical information applies to most hammer forged
barrels, it also tells of Ruger’s success in forging their own
barrels. Remember, too, that forging equipment and processes are
always being improved, and, as you read this, Ruger continues to
experiment with the goal of further improving their
barrels.
Introduction
When was the last time you read anything
complimentary about hammer forged barrels? Typically, you read
comments to the effect that the barrels are smooth, but not
dimensionally uniform, it’s just a fast way to make rough barrels,
they are not good for match barrels, etc. In fact, most American
manufacturers rarely even advertise that their barrels are hammer
forged. On the contrary, most European manufacturers do. Why the
difference? Why do European manufacturers tout hammer forging while
Americans hide it? Are forged barrels good for anything? In this
article, we’ll examine the process and answer these questions.
Brief History
As the most defining event of the 20th century,
World War II affected all aspects of life for all people involved,
but none more so than the shooting industry. WWII caused the
transition from bolt action to the assault rifles, single to double
action semi-automatic pistols, the death of revolvers in military
use, and the first significant use of portable, flexible machine
guns. Of the latter, the German MG34 and MG42 stand out for their
high cyclic rate – between 800 and 1200 rounds per minute. Barrels
heated up fast and quickly became worthless. To speed up production,
German engineers came up with the hammer forging process to pound
machine gun barrels to shape from the outside in. Interestingly,
Remington took the opposite approach when it perfected button
rifling a few years later by forcing the rifling from the inside
out. These two differences play a large part in the behavior of the
two barrel types which we’ll discuss shortly.
In the aftermath of World War II, forging expertise
ended up in Austria with GFM (http://www.agfm.com/ in the USA),
and they have become the leading hammer forging machine manufacturer
with machines dating back to 1946. European gun manufacturers began
using the technology shortly after the war while American
manufacturers didn’t start until the 1960s. Today, Sturm, Ruger
& Company uses 6 GFM machines to make all their centerfire
rifle, target rimfire, round handgun, and shotgun barrels. Remington
has more GFM machines than Ruger, and other manufacturers have one
or two machines each, some from other manufacturers. Hence, there
are about 20 hammer forging machines actively producing barrels in
the USA with none in the hands of small, custom barrel makers. The
machines cost over a million dollars each, so it is no wonder only
the largest firearms manufacturers have them.
Doing a little mental arithmetic, we can calculate
that the sales of GFM machines to American gun makers only amounts
to about $20 million over the past two decades or so, surely not
enough to keep a large machinery manufacturer in business. In fact,
barrel making is only a small part of GFM’s business; the automotive
industry uses many of these machines, especially in Europe. American
auto companies are starting to realize the benefits of hammer
forging, and more and more forged car parts make their way onto the
road everyday. While it won’t ever be as common as milling or
turning, hammer forging has slowly become a common process in the
manufacturing world.
The Hammer Forging Process
All rifle barrels start out the same way – a solid
bar of steel. Generally, makers use 4140 chromium-molybdenum, 410
stainless, or 416 stainless. Steel of these three alloys is widely
available from industrial suppliers, but barrel manufacturers go to
great lengths to work with the steel producers to insure uniformity
and cleanliness of the steel. That makes barrel steel much more
expensive to start with, but scrap savings more than offsets the
initial cost. The bars are then deep hole drilled (also called gun
drilling) and reamed. Hammer forging has a distinct advantage here.
Drilling small, deep, straight holes is time consuming and
expensive. That is why .17 caliber barrels cost more than larger
calibers. While button or cut rifling must start with a hole
somewhat smaller than finished size, hammer forged blanks have holes
about 20% larger than finished size. Furthermore, the hammer forged
blank is softer and only about half as long as by the other two
methods. Reaming the larger holes is easier as well.
Figure 1 -Computer
model of a hammer used on hammer forging machines.
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Figure 1 shows a “hammer” from a hammer forging
machine. While this hammer doesn’t look like the claw hammer in your
toolbox, it does indeed pound the barrel to shape. Given the
stresses involved with forging, these hammers are ground from solid
carbide with geometry much more complicated than can be seen in
Figure 1; hence, they are quite expensive. Those of you familiar
with carbide know it is quite dense. A set of four of these hammers
weighs about 40 pounds!
Photo 1 – .45 and .17
caliber carbide mandrels that form the
rifling.
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The hammers pound the barrel blank around a mandrel
as seen in Photo 1. The mandrel has a reverse image of the rifling
formed on its surface. The raised helical lines shown in Photo 1
form the grooves in the finished barrel. Since the forging machine
pounds the barrel blanks over the mandrel, the mandrels must be
ground from carbide and are also quite expensive. The mandrels
attach to the end of a steel mandrel rod (look ahead to Figures 2
and 3) providing the length necessary to reach through the barrel
blank. We would expect the mandrel rod to be smaller than the
mandrel, but that isn’t always true. The .45 caliber mandrel in
Photo 1 does use a smaller mandrel rod shown by the smooth, smaller
diameter portion on the right of the mandrel. However, the .17
caliber one uses a mandrel rod larger than the mandrel illustrating
how much larger the barrel blank bore is before forging.
Figure 2 – Isometric
view of the counter holder, hammers, mandrel, barrel blank,
and driver in the open
position.
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Now, we’ll look at how a hammer forger operates by
starting with some simple illustrations. In use, the hammers are
grouped in sets of four symmetrically around the mandrel as shown in
Figure 2. The barrel appears transparent so the mandrel rod can be
seen more easily. Note the counter holder and driver in that figure.
The counter holder positions the muzzle end while the driver pushes
and turns the breech end as the barrel blank feeds into the machine.
Figure 3 shows an end view of the four hammers, barrel blank, and
mandrel in the open position.
Figure 4 shows a section view of all parts in the
closed or hammered position while forging the first few inches of
the barrel. The end view in Figure 5 shows the hammers closed on the
barrel blank.
Figure 3 – End view of the
hammers, mandrel, and barrel blank in the open position.
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In operation, the barrel rotates as it feeds into
the forging machine and the hammers pound in unison between 1000 and
1600 beats per minute. The whole process takes between 2 ½ and 4
minutes depending on the caliber, length, and type of hammer forging
machine.
Now, let’s look at an actual GFM hammer forging
machine in operation at Sturm, Ruger & Company’s Newport, New
Hampshire facility. Starting close up, Photo 2 shows the forging box
with four hammers installed in the symmetrical pattern shown earlier
with the counter holder visible in the middle of the hammers.
Looking to the driving end of the machine, we see
the chuck head with the coned driver and protruding mandrel in Photo
3. The machine positions the driver and mandrel hydraulically.
Now, let’s step back and get the same view the
forging operator gets in Photo 4. The forging box with the hammers
operates in the left end while the chuck head with the driver and
mandrel occupy the right end. Those are barrel blanks in the
foreground of Photo 4.
Figure 4 – Section
view of the counter holder, hammers, barrel blank, driver, and
mandrel rod, in the closed
position.
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Stepping back a little further, we can see the
machine controller with the machine in the open position (Photo 5)
with the chuck head to the right and the closed position (Photo 6)
with the chuck head to the left.
Finally, Photos 7 and 8 show the work area from the
back side and the maze of hydraulics needed to control the
machine.
As a barrel leaves the machine, it has a curious
snakeskin pattern left by the hammers and the rotation of the blank.
Some manufacturers leave this snakeskin pattern as the final finish
while others turn the marks off in barrel contouring (machining the
final exterior dimensions on a metalworking lathe). Look at Photos 9
and 10 for a view of the breech and muzzle from a freshly forged
barrel blank made on the machine shown in Photos 2 through 8.
Figure 5 – End view of the
hammers, mandrel, and barrel blank in the closed
position.
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The forging process cold works the blank which adds
to the barrel’s hardness – the actual change in hardness depends on
the steel used. The .22 rimfire blank shown in Photos 9 and 10
started off as annealed 416 stainless and ended up about 23 on the
Rockwell C scale (Rc). Chrome Molybdenum 4140 steel starts off
between 19 and 25 Rc and forging hardens the barrel an additional 3
points. 410 stainless starts slightly higher at 20-26 Rc and may end
up as much as 30 Rc after the forging process. Given uniform steel
to start with, the hardness stays uniform throughout the length of
the blank. On the barrel shown, the breech and muzzle end agreed
within the normal tolerance of a Rockwell tester, about 2 Rc
points.
Hammer Forging Advantages
Photo 2 – Close-up of
the forging box with the hammers and counter
holder.
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While it seems like a rather crude process to beat
the barrel down on the mandrel, the process actually requires quite
a bit of finesse. Subtleties provide exceptional control of the bore
and groove dimensions. For instance, the mandrel is tapered and can
be moved in along the length of the barrel during forging. This
provides two advantages. First, by precisely locating the mandrel in
the bore, a specific bore size within 0.0001” can be obtained.
Second, by adjusting the mandrel’s position during forging, the
operator can create a tapered bore. Most riflemen know that having
the bore diameter at the muzzle slightly smaller than at the breech
helps increase accuracy while having the bore diameter at the muzzle
larger generally ruins accuracy. This is especially true when using
lead bullets. We’ll find there are other methods of tapering a
hammer forged barrel shortly.
Photo 3 – Chuck head
with the driver and
mandrel.
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Another advantage generally attributed to hammer
forged barrels is a uniform, smooth bore. A section from the barrel
shown in Photos 9 and 10 measured less than 20 micro-inches on a
Mitutoyo Surftest, and that is for an unlapped, production barrel.
See Figure 6 for output from the Surftest printer showing the nice
smooth surface. Output from a typical factory barrel would look like
jagged mountains on that printout. Hence, hammer forging provides
nearly lapped-barrel smoothness without lapping. Bore size depends
on the carbide mandrel to a large degree while uniformity depends
mostly on consistent steel. Given a good mandrel and clean, uniform
steel, the process carries the dimensions through the bore within
0.0002” on a production basis.
An Unusual Characteristic
Photo 4 – The hammer
forging machine from the operator’s perspective. Note the
barrel blanks in the foreground.
Photo 5 – The
controller with the machine in the open position, ready to
receive a barrel blank. The white arrow points to the chuck
head in the right or open position.
Photo 6 - The
controller with the machine in the closed position almost
finished with a barrel. The white arrow points to the chuck
head in the left or closed position.
Photo 7 – Rear view
of the work area.
Photo 8 – Rear view
of the forger showing all the equipment needed to operate the
machine.
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Most rifleman are familiar with the method of
creating button rifled barrels – the barrel maker pulls a button in
the shape of the rifling through a bore-sized hole, and the button
forms the lands and groves from the inside. Most accuracy riflemen
also understand that a button rifled blank must then be stress
relieved, contoured, and lapped for best accuracy. Lapping must be
the last step because any residual stress in the barrel will cause
the bore to open up during contouring. Since most barrels taper
smaller at the muzzle, the residual stress causes the bore diameter
at the muzzle to open up – exactly the opposite of what we want for
accuracy. In fact, the increase in outside muzzle diameter for the
last few inches of Anschutz target barrels has long been thought to
increase accuracy by creating a choke effect. Since this part is not
turned down as much as the rest of the barrel, the bore diameter
stays smaller. This characteristic of button barrels can be summed
up as: the smaller you turn the outside down, the larger the bore
becomes. Please understand that we are not being critical of the
buttoning process. All barrel making processes have advantages and
disadvantages, and the barrel makers learn to work with the process.
How well they learn to work with the process determines the quality
of the barrels. Makers such as Shilen, Hart, and many others have
long mastered the buttoning process and produce match-winning
barrels.
Photo 9 – Breech end
of a forged barrel blank.
Photo 10 – Muzzle end of a forged barrel
blank.
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Hammer forged barrels perform opposite from button
barrels. Since hammer forged barrels are formed from the outside in,
when we contour the barrel on the outside the bore gets smaller.
Many of you readers have already figured out, then, that by
contouring a hammer forged barrel with a normal barrel taper, the
muzzle bore diameter is smaller than the breech bore diameter, the
most desirable condition for accuracy. In fact this effect is most
pronounced on large caliber, light barrels. When making blanks for
those types of barrels, the forgers typically forge the blanks to
the upper Sporting Arms and Ammunition Manufacturers’ Institute
(SAAMI) bore specifications so that the contouring process will
leave the muzzle bore diameter within the lower SAAMI specification.
Not too many people benchrest test a 6 pound .308 Ultralight, but
the tapered bore should help accuracy. For varmint weight barrels
which aren’t turned down much, the difference is not very
measurable, but you can feel it by pushing a soft lead slug through
the barrel. Oddly enough, most hammer forgers don’t stress relieve
the blanks after forging. Many tests have been run, and no
significant accuracy difference has been noted between stress
relieved and non-stress relieved barrels, at least not in factory
sporter or varmint rifles.
Summary
Figure 6 – Surface
roughness of a hammer forged barrel.
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Hammer forging was developed in Europe and has been
used there for about 30 years longer than in the US, so forged
barrels are widely accepted and advertised by European gun
manufacturers. In the US, hammer forging has long been considered
one of those black arts of the gun making trade, and the fact that
only a few dozen people nationwide actually use this machinery
compounds the mystery (eight people make ALL of Ruger’s forged
barrels). However, as we have attempted to show in this article,
hammer forging makes dimensionally uniform, smooth barrels which
tend to err on the side of a properly tapered bore, and the process
makes barrels quickly. Because of all of these advantages, most
round factory barrels, including shotgun barrels, are hammer
forged.
Why don’t custom gunsmiths use hammer forged
barrels? Startup costs. The million dollars per forging machine
price tag deters small users. And, since the factories typically use
their entire production capacity internally, custom gunsmiths simply
can’t get hammer forged blanks easily. Were they available, we think
they would be well received based on their performance. In future
articles we’ll look at experiments with hammer forged barrels that
demonstrate their accuracy potential in custom rifles.
Acknowledgments
Photos 1 through 8 were provided by Ed Thorson of
the Pine Tree Castings Division of Sturm, Ruger and Co. Photos 9 and
10 were provided by Harry Higley. Mark Gurney, the Engineering
Manager at Pine Tree Castings, introduced the two authors and
suggested this article. As a division of Sturm, Ruger and Co., Pine
Tree Castings investment casts Ruger parts as well as parts for
outside customers, including other firearms manufacturers.
About the Authors
Vern Briggs is the Forging Process Engineer at
Sturm, Ruger & Company, Newport, New Hampshire where he is
responsible for barrel production.
James Higley is a Professor of Mechanical
Engineering Technology at Purdue University Calumet, Hammond,
Indiana where he teaches courses in design and manufacturing.
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