last part of the firearm touched by the bullet before it exits. As the bullet clears the crown, it will release upwards of of back pressure, which must be as uniform as possible. While many barrel manufacturers recess the crown to protect it from accidental damage, it can still be damaged over time by cleaning rods that are too hard. It is also not uncommon for factory crowns to be cut slightly off-center, so that one side of the bullet exits slightly earlier than the other, and this will result in the bullet being pushed away from that side, causing a significant deflection in its path. A crown may be re-cut with relative ease, and this can fix any problems caused by a defective or damaged crown, by ensuring an even release of the bullet. Stress Any machining process on a barrel, whether it's boring, rifling, or turning the outside contour, will produce some stress in the steel of the barrel. This stress can cause the barrel to expand unevenly as it heats, causing shots to "walk" as the barrel heats and cools. To prevent this, careful post-machining heat treatment is often used to stress-relieve the barrels. The amount of good this does depends on the technique used to make the barrel. For example, the hammer forging method of manufacture leaves a significant amount of stress in the barrels, which could be addressed through stress-relieving heat treatment. Wear Barrel wear is also an important issue, particularly in high-powered firearms. High temperatures tend to erode the barrel at the throat, preventing the bullet from entering the rifling cleanly. One way to produce a long-lasting barrel is by the proper selection of materials. Stainless steels, such as 416, have been shown to have a longer life than the traditional 4140 chrome/molybdenum steels used for barrels. While the stainless barrels are not any more accurate than a 4140 barrel, they will maintain their accuracy longer in many applications, as they are more resistant to erosion caused by the heat of firing high-powered cartridges. One notable exception to this is the .50 BMG cartridge; competition shooters often fire high-precision lathe-turned bullets made from harder brass, bronze, or steel in this caliber, and the 4140 steel will stand up to this better than stainless. Cryogenic treatment Another commonly cited accurizing treatment for barrels is cryogenic treatment. This involves slowly cooling the steel down to liquid