I talked to the barrel guy at
Delta Gun Shop in Colville Washington and he explained the three methods for rifling barrels.
Button Rifling: push/pull a "button" through the bore. This is NOT a cutting process, but more of a metal displacement process. This introduces stresses into the barrel which must be relieved before the barrel contour (outer shape of barrel) is cut. Also, after the stress relief, the bore diameter will change slightly, so this must be accounted for in advance.
Hammer Forging: Didn't really talk about his, but apparently HK uses this method. Also NOT a cutting method. This is a hot forming method and has issues similar (but opposite) to button rifling in bore diameter changes.
Cutting (for lack of a better term): Run a tool down a cold bore that cuts a single groove 1/10 of a thou deep. Index bore, cut next groove. Cut all grooves and then increase depth of cut another 1/10 thou. Do this repeatedly until you are done... apparently takes about an hour to cut the rifling on a barrel. I got to see the guy's setup and he showed me how it worked. Pretty neat! Introduces no internal stress in the barrel as it is a cold process.
For the record, the guys at the Delta Gun Shop kick butt! I needed a place to store my guns while on a trip to Spokane because we made a detour into Canada to see my wife's mom. I asked Jim if I could store my guns with him for the week for a modest fee. He replied that that would be just fine. When I got there, the "modest fee" turned out to be $0.
Jim is a custom gun maker, specializing in bolt action rifles, so it seems (they do a lot of other stuff too) and his rifles look pretty great. I think when I have more cash, I'll turn to Jim for a custom rifle. Great guy.