Re: cold rotary hammer forged vs. cut rifled?
Here ya go:
Hammer Forged Barrels by
Vern Briggs and James Higley
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 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!
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.
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.
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.
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.
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
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.
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
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.
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
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.
