In the begining is a round steel bar that will become a barrel.
The barrels start as a round bar of steel, several inches longer than the ultimate length of the finished barrel. The steel bar is drilled to a diameter slightly larger than the desired bore diameter. Then, to create the rifling in the bore, the steel bar is inserted into a hammer forging machine. The FNM plant has four of these machines in production. They are incredibly expensive compared to the equipment required to do button or cut rifling. But once in place hammer forged barrels (often called swaged barrels) can be extremely precise, durable and accurate.
Now let me go back for a minute. Before inserting the barstock into the machine the oversized bore is fully honed to a smooth polish. This eliminates any roughness that may be left over from the drilling process. It is how you can obtain a smooth, extremely precision bore surface using the hammer forge process. This is a key new Model 70 advantage.
With the honed barrel inserted into the machine a full-length mandrel is inserted into the bore of the bar of steel. This mandrel is an exact reverse image of the desired rifling and lands impression you want on the inside of the barrel. With the mandrel inserted the machine is put in motion and massive “hammers” numbering three or four pound the exterior of the barrel forming it against the mandrel. It pushes with so much force that the steel actually flows to the mandrel image forming sharp rifling corners and remaining smooth and clean inside. This can take up to two minutes to complete depending on barrel length and type and the number of blows to the barrel surface -- which is often 6-7,000 blows per minute. It is an awesome sound -- pounding, pounding, pounding. It is process that is somewhat oily and messy, but it works extremely well. When completed, the hammering is so consistent from end to end that it still appears as if it is a smooth, polished surface. Yet inside a nearly perfect image of rifling and lands has been formed and the polish of the honing process has also been retained. With a little hydraylic force, the mandrel is pulled from the bore, turning with the rifling as it comes out and the barrel is ready for the next procedure on its way to becoming a Model 70 barrel.
Not only does swaging the rifling create a superior rifling inside the barrel, but it also alters the metalugy slightly as the steel is hammered until it flows. This process can increase the strength of the steel in the bore, giving you improved barrel durability and smoothness. This durability and smoothness are significant contributing factors to reaching the potential for Minute of Angle accuracy . . . something that comes with every Model 70.
In the hammer forgin shop there is one more thing to do: The forged bar stock is placed in racks and it leaves the hammer forging area – being wheeled into the barrel shaping room. Just a little skinnier and just a bit longer than it was when came to the plant, but now it's ready for the next step.