A European website has a reprint of an excellent article by Vern Briggs of Ruger and Professor James Higley of Purdue. We’ve discussed the various ways of rifling barrels; we thought you’d appreciate Briggs’s and Higley’s deep dive into the process and technology of the most capital-intensive form of barrelmaking, cold hammer forging. (Actually, it could be hot hammer forging just as easily, as we’ll see at the end).
They begin with a history lesson:
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.
As far as we know, the first use of hammer forging in the USA was by TRW on the US Rifle M14 contract. TRW was selected, in part, because it wasn’t a firearms manufacturer, but instead was a maker of machinery and aeronautical and automotive parts. Ordnance officers thought that TRW might be able to bring down costs and improve quality by applying automotive mass-production technology — and that’s exactly what they did with hammer-forged barrels.
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 precision achievable with these machines is almost otherworldly.
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.
This was how the German war industries created the Gerlach taper-bore or squeeze-bore weapons during World War II. In essence, they used a tapering (but rifled!) barrel to squeeze down the driving bands on high velocity kinetic-energy rounds (with tungsten-carbide penetrators).
Here is how Daniel Defense makes an AR barrel, starting with steel rod, drilling a pilot hole, gundrilling the bore hole, then running it on the GFM machine, profiling it, chambering, etc.
One of the most interesting scenes (to us, at least) was the toolmaker using a surface grinder to reconfigure and restore the worn faces of hammers. The hammers last about 1,000 barrels before needing maintenance.
American GFM corporation links to a number of videos of these machines in operation. Here’s a sub-5-minute video of how a gigantic rotary forge machine takes a steel tube and forms it into a cannon or tank main gun barrel. It’s just like the Ruger or Daniel Defense process, except much larger — and the barrel preform is heated to roughly 2000ºF and maintained at that heat while being forged.
The Army designed and built its own machine, but it’s clearly a kissing cousin of the GFM hot-forge process.
The strengths of this process are speed and consistency. And the biggest obstacle to using this technology, of course, is the barrier to entry: such a machine is extremely expensive, even if you don’t need one big enough to work on 8″ guns.