between moving parts must be kept to the smallest value that will allow proper operation of the mechanism. This goal can be achieved by carefully hand selecting parts and precisely fitting them together, or by manufacturing new parts (bolt, receiver, barrel, etc. to precise dimensions using much tighter tolerances than production components. The best fits are generally attained by selecting slightly oversized parts (or altering stock parts to form an interference fit) and then lapping the mating surfaces to attain the desired fit (clearance). Clearances cannot be too tight, however, or functionality will be compromised; this is very important in automatic and semi-automatic firearms, where there is a certain amount of energy extracted from the firing of the cartridge that must be used to cycle the action. Overly tight clearances mean that there is no place for lubricant and dirt to be, and this can bind the parts. However, except for cost considerations, it is always beneficial to minimize the tolerances applied to the manufacture of mating parts. In some cases, stock parts are not sufficient for the task of producing a finished product with sufficiently tight clearances. In this case, it may be necessary to use specially fabricated parts, built either with minimal clearances (but to a very tight tolerance), or parts that are oversized and intended for hand fitting. Barrel The barrel is one of the most important factors for accuracy, as a poorly made barrel may be beyond correction. Even a quality barrel must be well matched to the cartridge it will be firing. In most cases, it is not practical to fix a barrel whose bore is worn out, poorly or unsuitably rifled, or the wrong bore diameter; the primary exception to that is rimfire barrels, which can be inexpensively bored out and re-lined with a commercial barrel liner. If the barrel is unsuitable and relining is not an option, then an aftermarket or custom barrel is the best solution. However, if the bore is good, then there are a number of operations that can be done to the barrel to improve upon its accuracy. Bore Ideally, the bore must be cylindrical and the rifling geometry the same down the length of the bore. Some airgun rifles have a short cone towards the muzzle to improve the speed of the bullet. A slight gain in the rifling twist, or a slight tapering down of the bore,