nitrogen temperatures, leaving it there for a time, and then slowly warming back to room temperature. This process converts remaining austenite in the steel to martensite. Many proponents of this process claim increased accuracy of the resulting barrels, but independent testing of the process by major manufacturers has shown no increase in accuracy. However, the conversion of austenite to martensite has been shown to result in easier machining and greater wear resistance on steels that tend to have significant amounts of retained austenite, such as stainless steels, and the process does appear to significantly affect the accurate life of stainless steel barrels. Rifle stocks A poor fit of the action to the stock is also a source of problems, and this problem is exacerbated by issues such as thermal expansion of metal parts during use, and swelling and contracting of wooden stocks with changes in humidity. These changes can affect accuracy either by allowing the action to shift under recoil, or by causing slight but accuracy-destroying bending of the barrel. By removing wood from areas of contact and, when needed, replacing it with a more stable substance that is precisely fitted, such as a fiberglass composite molded in place, a more stable, precise fit can be achieved. Other materials, such as composites or laminated wood, can also provide a stronger, more dimensionally stable stock than traditional woods can. Some stocks are even being made out of aluminum or other metals, for maximum stability. The process of fitting the action to the stock is called bedding, and there are a number of different processes used. Glass bedding uses a fiberglass composite which is molded in place around the action. This can be as simple as bedding the recoil lug of a bolt-action rifle, or as complex as bedding the entire action and barrel channel. Pillar bedding uses precisely machined metal pillars in the stock which mate with the action, providing a strong metal-to-metal contact without the labor-intensive hand fitting required in glass bedding. Many times it is advantageous to eliminate most or all contact between the stock and barrel, to eliminate potential accuracy-destroying interference in the barrel harmonics. To do this, stock material is removed along the barrel channel to leave a slight gap between the stock and the barrel; this is called free floating the barrel. With some designs, pressure bedding is preferred, where a single point of contact remains