The button is actually a tool made of hardened steel or titanium carbide. It looks something like this:
The button is essentially a negative image of the barrel, i.e. the grooves to be cut are carved in relief on the button surface. There are two ways this button can be used: It can be pushed into the barrel or it can be pulled through.
The machinery to cut the rifling is very simple. The button tool is attached to a rifling head, which is in turn attached to a long rod of high tensile steel and the other end of the long rod is attached to a hydraulic ram. The rifling head is capable of rotating the button at a fixed rate, to produce rifling at a given twist rate as the button is being pulled or pushed through the barrel.
A Button Rifling Machine
In the above picture, you can see a barrel mounted in the middle of the machine. The right side of the barrel is blocked by a thick steel plate and on the left side, you can see the button about to be pulled into the barrel from left to right. On the right hand side, the pull-rod for the button is attached via a twisting spindle to a frame that has two hydraulic rams attached to it. Also on the right side is a rack and pinion gear system that drives the twisting spindle and twists the button as it is pulled through the barrel
The bore of the barrel is lubricated first, before the button is pulled through it. Each manufacturer has their own secret sauce as to what lubricant is used, and this is kept a closely guarded secret. As the hard button passes through the softer steel of the barrel, it engraves the rifling on to the inside of the barrel. The whole process takes only a minute or less. This is a cold-forming process, so there is no heat, but there is a lot of pressure involved (60,000 psi or 410,000 kPa is not uncommon), which is why this process requires a hydraulic ram. As you can imagine, this involves putting quite a bit of stress on the barrel, hence the barrel must be stress relieved after the grooving operation is completed.
The simplest types of rifling buttons merely cut the grooves into the barrel, and so these leave some burrs in the barrel after the grooves have been cut. Hence, the barrel needs to be finished separately if these cutters are used. Some other rifling buttons are combo buttons that have both a rifling button and a finishing button or sizing button following it. The rifling part cuts the grooves and the finishing part pushes the burrs back in and straightens out the edges caused by the rifling part of the button.
In the above image, we have two rifling buttons. The top one is a combo button which has a rifling button as well as a sizing button and the bottom one is a simple rifling button.
One school of thought believes that a pull button has a tendency to break off inside the barrel while it is being pulled through and hence, they use a push-button technique. In this case, the push rod is supported as it enters the barrel, to prevent buckling. The other school of thought believes that a pull button is best because it stays straight and true through the pulling operation, whereas with a push operation, the button has a tendency to yaw and cut a non-uniform thread. Needless to say, each school of thought has its own party of supporters.
The button must be made slightly larger than the desired bore and groove diameter of the barrel because steel is an elastic material and tends to spring back when it is pushed or bent. Hence if a groove of say 7.8 mm. diameter is sought, the grooves on the cutter would actually be slightly more than this (say 7.85 mm. groove diameter or so). Obviously, the properties of the steel used from batch to batch must be more or less uniform, otherwise the spring-back amount may be more or less and a new button will need to be made to compensate for this. Another interesting thing about this process is that it must be done when the barrel is in a blank state (i.e.) the barrel is an uniform cylinder. This is so that the forces resisting the button's movement are uniform throughout the barrel. Otherwise the rifling will be uneven inside the barrel.
As mentioned earlier, after the grooves have been cut, the barrel must be stress relieved because the process puts a lot of stress on the steel. If this is not done, the barrel may split or deform later when the weapon is fired. Another reason to do this is because after the grooves are cut, if the barrel is further machined on one side (e.g. the muzzle end is slightly thinned and it is machined to fit the chamber), the barrel will deform on that side due to the stresses from the button rifling operation, if the stress isn't relieved first.
Stress relieving is done by heating the barrel in a furnace to about 525-550 degrees celsius and then slowly cooling it. The bore may slightly snap back during the stress-relieving process, which is why the button is slightly larger than the desired size. Even though the barrels have been stress-relieved, it is not possible to do it completely without compromising the hardness of the barrel's steel. Hence, when the barrel is machined again, the bore may open up slightly on that side, even if the machining is done on the outside of a barrel. Barrel makers usually use a process called lapping (which we will discuss later when discussing the art of barrel making) to restore the barrel back to a uniform bore.
There are a lot of variables that affect the bore and groove dimensions with button rifling: the particular batch of steel involved, its hardness, size of the bore, size of the button, how fast the button is pulled through, how much the bore closes after stress relieving and how much it opens after final machining etc. This sounds like a lot of things can go wrong, but a skilled and experienced barrel maker knows how to balance all the factors out to make the correct dimensions. Still, at the end of the day, the barrel maker only knows what the final exact dimensions of the bore will be, after the barrel is fully machined. This is why some barrel manufacturers offer different grades of button-rifled barrels.
Button rifling is very common among most barrel makers in the United States. In fact, it is the most used method today, because of the small amount of time needed to make the rifling, and the fact that if the barrel-maker is experienced, the dimensions of the final product can be very accurate as well. The tooling and machinery used to produce button rifling is also very cheap. This is why this method is used a lot in mass production.
On the other hand, the disadvantages are that the material of the barrel needs to be very homogeneous and uniform hardness throughout the bar, otherwise the cut will be uneven in the softer parts of the barrel vs. the harder parts. The exact twist of the rifling is also somewhat unpredictable. If the button should slip inside the barrel, you may get a barrel with (say) 1 turn in 205 mm. when the desired rifling is 1 turn in 200 mm. Another problem is that the groove may not be centered (i.e.) one side might have deeper grooves than the other side due to non-uniform hardness of the barrel.
Compared to the other two methods we studied earlier, this method offers more disparate results. This is why manufacturers offer different grades of button rifled barrels. The ultra premium quality button-rifled barrels cost a lot more than the standard and sporting quality button-rifled barrels.