We know that our Bulgarian rifles (and others), have their barrels manufactured in this manner, so I figured that the following basic history and description of the process would be enjoyed by some of you here:
HAMMER FORGED RIFLING:
The technique of hammer forging rifle barrels was developed by Germany before WW2 because the MG42 machine gun, with 1200 rounds per minute rate of fire, positively ate barrels. The first hammer rifling machine was built in Erfurt in 1939. At the end of the war it was shipped down to Austria ahead of the advancing Russian army, where American technicians were able to get a good look at it.
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 Steyr, Austria. They cost about a half a million dollars and can spit out a barrel every three minutes. These machines have reached a very high degree of development and are so sophisticated that they will not only hammer the rifling into the barrel, but it is also possible to chamber it and profile the outside of the barrel all in the one operation. Only large scale arms manufacturers and ordinance factories have pockets deep enough and barrel requirements insatiable enough that they can afford to buy and run such a machine.
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.
Hope you all enjoyed the read :)
Never knew the origin and motivation for the technique.
Glad you found it interesting :)
Just to add a bit more to this subject, here is some more info you may find worth while reading:
The other two well known methods of barrel rifling are the "cut" and "button" rifling.
Both of these methods can produce a more consistant barrel in the accuracy dept (with the "cut" rifling probably having the edge over the other).
With all else being equal, the forged rifling process is usually known to produce a less accurate barrel than the other two methods. It also has been seen by some as a cheap and fast way to produce a barrel, while offering no advantages.
With that being said, here are some other things one should know about the forged rifling process:
While some think it is the cheap way to go, it actually is the most expensive method to get involved in. The cost of the machinery (over $500,000 per machine), and the higher cost of maintenance, makes them accessible only to companies that have the money to invest in them. Once they pay for themselves, they can save money when doing larger production runs, but again, the initial investment is a pretty big deal.
Though it is true that when all else is equal, the hammer forged rifling will usually produce a slightly less accurate barrel than when using the other two methods, the difference is not big enough in most cases. While it may not be the best method to produce a competition barrel, it is a great method for the typical military one. Why?.........because the hammer forging produces a more dense steel, therefore making the barrels steel (including the rifling) more dense. The immediate advantage would be that the rifling will be more resistant to wear, and therefore last longer.
This benefit will be more evident in firearms like the Glock pistol. Why?........because their barrels have no lining (hard chrome or otherwise), and having the denser barrel steel will add to the barrel's life (when compared to the other two methods of rifling).
The advantage the hammer forged rifling has in an AK may be less significant. Why?.........because AK's usually have a hard chrome lining, which is much harder than the already hard steel it is plating. With this being the case, the bullets will not contact the barrel steel, instead it will contact the very hard chrome plating in the barrel. Though there still may be some advantages to the barrel steel being more dense, it is harder to see where the advantages would be in a chrome lined AK barrel.
It definitely makes for tougher steel when a barrel is made using the hammer forged rifling method.