You should read this Rob...
HAMMER 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. Also, because the metal is worked completely throughout the barrel there are considerable radial stresses induced which are difficult to remove completely by the usual stress relieving methods. Stainless steels tend to work harden to a much higher degree than Chrome Molybdenum steels and so do not remain malleable enough to hammer forge. Because of this, it is difficult to make stainless barrels this way. Stainless barrels are being hammer forged, but using type 410 steel which has a lower chrome content than the regular 416 steel usually used for making barrels by other methods.
Most of the big hunting rifle makers in Europe hammer forge their barrels. Sako and Tikka in Finland, Heckler & Koch, Steyr and Sauer in Austria. Now, Ruger in the US has started making barrels using this method.
The ability to relieve stress in hammer forged barrels is virtually perfected and this is evidenced by the sub MOA accuracy of these European made rifle barrels.
The new Rugers are certainly no slouch in the accuracy dept. either.
