Can stainless steel barrels be cold hammer forged?

In recent years, it seems that firearms enthusiasts have grown obsessed with having cold hammer forged barrels on their rifles. This movement is one that I’ve noticed throughout the gun community, but it comes across especially strong among AK fans. Touted as stronger than  alternatives, hammer forging is a process that quite literally hammers the barrel into shape (rifling included) and it’s often seen as the pinnacle of manufacturing methods. While this may well be the case, are we missing important details while obsessing over how our barrels were made?

The video embedded below was put together by AKFiles forum member, Mel64D. In it he tests the Rockwell C-scale hardness of several hammer forged AK barrels from countries like Bulgaria, China, Poland, and Romania. Next to these (former) communist bloc parts, he also features US-made, 4140 and 4150 CMV (Chrome Moly Vanadium, additives to the steel) barrels from several American retailers. For the too long, did not watch crowd, I’ve posted the final numbers below the video.

Wrapping it Up

Some readers may be surprised by the results. The US-made barrels are nearly as hard (or harder) than hammer forged, military spec parts from countries that have been doing it for years! How is this possible? The key here appears to be the material the barrels are made of, not the manufacturing process.

Unlike the American CMV barrels, many enthusiasts feel that the hammer forged versions are made of simpler (or at least different) carbon steel, and a few versions at that. By itself, the imported carbon steel appears to be slightly softer than CMV 4140 and 4150. After work hardening via the hammer forging process, the military spec barrels seem to approximately match the American CMV alternatives. All else being equal, hammer forging produces a harder barrel. But all is not equal if the steels used in different parts of the world are, in fact, different – and they are.

This brings us to another consideration. We aren’t getting the full story from the US-made barrels, at least as far as the Green Mountain barrel is concerned. Green Mountain barrels are (mostly) produced using a cold-forming method called button rifling. To add rifling to the barrel, Green Mountain pulls an extremely hard button (with the rifling negative) through the barrel. This button moves steel to create rifling rather than cutting it. Thus, the bore of a button-rifled barrel undergoes work hardening similar to forging. Mel64D tested the outside of the barrels, capturing the work-hardended surface of the forged parts, but not the hardened bore of the button-rifled tubes. We should limit conjecture here, but it’s reasonable to say that the most important parts (the bores) of the CMV barrels may actually be even harder than the values shared here.

Now observant readers have probably noticed that I have yet to mention additional details like the presence of chrome lining or nitride finishes. For this discussion, we are going to ignore such enhancements. Generally speaking, the process to chrome line a barrel is the same, regardless of origin or manufacturing style. Chrome lining results in a harder bore, but if the underlying steel is soft, its advantages are diminished. None of the barrels in Mel64D’s test were nitrided, so the advantages of the process are fairly irrelevant at the moment.

While it is always possible that Mel64D’s test setup was improperly calibrated, his methodology looks to be sound. It’s also fair to question whether hardness is everything when it comes to barrel toughness. There’s no doubt that hardness is touted as an advantage enjoyed by hammer forging and Mel64D’s video isn’t intended to dispute that claim. Rather, his findings seem to confirm that we often get far too wrapped up in how barrels are made, sometimes forgetting that the materials that go into them are equally important.

August 30, 2022 2498 view(s)

Cold Hammer Forged Barrels: How They're Made and Are They Worth It?

The science poured into modern gun barrels is staggering, and lightyears ahead of the smoothbore muskets used by most patriots during the Revolutionary War. Today’s enthusiasts almost take that interior rifling for granted, despite the fact it imparts spin to the bullet, stabilizing it in flight and tightening groups at long distance. Creating those interior lands and grooves is no easy task, but when it comes to the process that delivers performance, reliability and longevity—without breaking the bank—cold hammer forged barrels are hard to beat.

There are five major methods of creating rifling. As technology marches on there will likely be more, but an examination of the other techniques helps underscore the advantages of a cold hammer forged barrel.

Methods of Rifling 

Cut Rifling

This is the oldest technique used and, as you might expect from its age, labor intensive and expensive. In this process a hook-shaped cutter is pulled or pushed through the barrel while simultaneously rotating at the rifling twist rate.

Each pass creates a single groove and, because there is always more than one, the cutter must go through multiple times. Alignment must be precise for each pass. Relatively little stress is imparted on the barrel, but lapping follows to remedy the minute imperfections common to cutting. It’s another step in an already lengthy and labor-intensive process. 

When done right tolerances are squeaky tight, making them a popular choice in the serious long-distance shooter crowd. It’s not a procedure that lends itself to novice hands, however, or volume production. It takes skilled craftsmanship and tedious attention to eliminate performance-robbing variables, and as a result they are expensive. Experienced enthusiasts capable of printing mind-bogglingly tight groups at staggering distance may reap the full benefits, but today’s cold hammer forged barrels also win matches.  

Broach Rifling

Broach rifling is similar to cut rifling, except grooves are created with a single pass of a metal bar wearing multiple cutting blades. The cutters are progressively higher in each “row,” with the last one gouging each of the grooves to their final, prescribed depth. The bar, obviously, must also rotate at the proper rifling rate for the barrel.

The process was once a popular, timesaving one for mass-produced firearms. It’s taken a back seat today because holding barrels to precise tolerance with this technique is a real challenge and the results are rarely match-grade. It’s still in use on some handgun barrels, however.

Button Rifling

A carbide tungsten button—really a bullet-shaped tool with the rifling pattern reversed and milled on its surface—is pushed or pulled through a barrel in button rifling.  As it passes through the smooth barrel bore interior material on the sides flattens. For utmost in performance, stress must be relieved later. 

Done right the results can be match grade, but those buttons are not cheap. They are also designed for a specific caliber and permanently affixed at a single rate of rifling. It’s a costly investment for manufacturers—one passed onto enthusiasts.

Cation Rifling

Leave it to chemists to create a barrel-making process using acid. A rod pushed through the barrel’s bore deposits acid where grooves are etched—at the proper rifling rate. The solution is allowed to dwell long enough to etch/remove metal and, depending on the preferred groove depth, it can require multiple applications.

The approach is still in its infancy and the jury’s still out on consistency, although early performance is encouraging. Variables include temperature, slight variation in steel alloy from lot to lot and different factors. It can, however, process material that defies other methods. Odds are good we won’t see cation rifling soon due to the expense, as well as the added challenges of hazardous material handling and storage.

Cold Hammer Forging

A cold hammer forged barrel starts life, so to speak, as a short and fat blank with a polished hole running through the center. A hardened mandrel of proper width (caliber) and rifling pattern—again reversed/inverted—is placed into the smooth bore. The pair go into a forging machine that compresses the steel against the mandrel, hammering it into final shape. The barely detectable external spiral patterns that remain on some barrels are imprints from those hammers at work. Some companies polish them out, however.

Cold hammer forges are also a significant investment for companies. However, when the process is done and the mandrel removed, the resulting barrel is the proper length and profile with rifling that is consistent and butter smooth. No lapping is required—a time and expense saver—although a stress-relief step often follows.   

The process takes place at room temperature, despite the misleading “cold” terminology. There is a hot hammer forging process, but the equipment is even more expensive and the slight improvement in grain consistency hasn’t proven advantageous enough for a return on that investment.

Performance is impressive and consistent. Even serious long-distance shooters admit it’s not uncommon to encounter cold hammer forged barrels that perform close to or on par with good cut rifled versions. They outperform the skills of nearly every gun owner, especially when it comes to AR-15 chamberings, where the primary mission isn’t usually stretching the distance to 500 yards and beyond.

One of the biggest advantages of a cold hammer forged barrel is longevity, though. They thrive in nasty environments, survive abuse and last longer. The manufacturing process adds those enviable virtues with each strike of those hammers—strengthening the metal in a work hardening process employed by blacksmiths for centuries.

A chamber can be formed and barrel contour shaped at the same time. It’s a versatile approach that produces some of the most consistent barrels available today.

Which One’s for You?

If long-distance is your passion and you home brew custom cartridges to print tiny groups in the next zip code, cut rifling is the optimum choice. Those who spend weekends punching paper at 1,000 yards know well, though, putting one on a favorite bolt gun is going to be an investment. Even with ideal pampering its lifespan will be a relatively short one in firearm years. Button rifling is a solid second choice, but not always the Holy Grail long-distance shooters dream about.

Cold hammer forged barrels, however, last longer and perform beyond the abilities of most AR-15—even AR-10—cartridges. High-volume shooters don’t need to worry about burning out an expensive barrel and, even if your passion is precision with your modern sporting rifle, the only detectable real-world difference you’ll experience is price tag.

[Bear Creek Arsenal recently invested in 2 state-of-the-art cold hammer forged machines in order to provide our customers with even better quality products at phenomenal prices. Check out our first selection of hammer forged products below!]

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