How are barrels rifled? - Part 3
by B.B. Pelletier
Hammer-forging, also called rotary forging today, is the high-rate production way to make a rifle barrel. The process takes three to four minutes, start to finish.
The hammer-forging process itself is actually close to two centuries old. It was used by gunsmiths in Appalachia, who hand-hammered steel barrel blanks around mandrels to make barrels for the famous Tennessee Poor Boy rifles of the 19th century. They used a special "gun anvil" that had a special hammer fitted to it. The barrel blank was placed between the hammer and anvil. The special hammer was struck by a conventional hammer to form the steel.
A modern hammer-forged barrel is formed by beating on a steel tube with multiple hammers until the steel forms itself around a tungsten carbide mandrel. Believe it or not, the correct alloy of cold steel will flow like toothpaste when the circumstances are right. The mandrel is engraved with a reverse of the rifling pattern. When the steel conforms to it, it takes on the exact pattern the maker desires. The hammers strike the steel tube about half-million times during the forming process, and they leave a pattern on the outside and inside of the barrel. Some companies remove this pattern, while others, like Mannlicher and Ruger, proudly show it off.
Ruger proudly displays the marks of hammer forging, seen in this enlargement of their 10/22 Target bull barrel as spiral flats running the length of the barrel. The marks running across the flats (top to bottom in this picture) are tiny grooves in the steel that opened up during the forming process. You can see these rough marks only under magnification and the right lighting.
The blank steel tube is fatter and about 30 percent shorter than the finished barrel will be. When the barrel is finished, a lot fewer operations need to be performed than with barrels rifled by other methods. The bore comes out glass-smooth and very hard from the hammering process. That hardness resists erosion from the heat of the combustion gasses of a firearm cartridge, thus prolonging the life of the barrel.
On the negative side, hammer-forging introduces stress points that must be relieved or the barrel will warp when it heats up. So, target shooters don't pick hammer-forged barrels. It isn't as critical for rimfires as it is for centerfire cartridges, so Ruger gets away with hammer-forging its 10/22 Target barrel. The only way a centerire rifle can get away with one is to have a lot of stress relief after the barrel is made.
Of course, airguns produce no heat to speak of when they fire; so, the barrel stress points are not critical, and target air rifles and air pistols are a possibility. We know that IZH makes hammer-forged barrels, and there are probably other companies who do as well.
Why doesn't everyone do it?
A new hammer-forging machine costs $750,000 or more, making it a large capital investment. When a button-rifling setup costs $50,000, or so, it's hard to justify the more expensive process unless the demand for barrels is very high, as each barrel has to carry an amortized share of the machine's cost. Only companies making thousands of barrels per month can afford to do it this way.
There is no particular advantage to a hammer-forged barrel on an airgun, nor is there any disadvantage. If a maker can afford a machine, it can pump out a lot of barrels, but there has to be a market for them.