will still ensure the tight fit of bullet to bore, so when selecting a barrel, the tighter or faster twist end should be chosen for the muzzle. The twist of the rifling must match the intended ammunition for best accuracy. Rifling with too slow a twist will not stabilize long bullets, causing them to precess in flight; at its worst, this can result in the bullets tumbling in-flight and keyholing, where bullets strike the target sideways. Too fast a twist can also be a problem, as it can magnify problems in the bullet. A bullet whose center of mass is slightly off-center will diverge at a rate proportional to the rifling twist, so excess twist will result in greater dispersion. In practical terms, this is only a problem for rifles chambered for common military calibers where a variety of different loads exist. For example, the M16A1 rifle is unable to accurately fire bullets that are heavier than due to barrel twist that is too slow to stabilize heavier bullets. Precision rifles typically come with barrels that are either tailor-made for a specific ammunition load or made according to the buyer's specifications. Barrels can also benefit from lapping, not only because it makes the bore a more consistent diameter, but also because it polishes the bore. Barrel lapping should be done with the tool moving in the same direction as the bullet will move, so that any imperfections in the barrel will be smoothed out and thus not interfere with the passage of the bullet. A smooth, polished bore will not only hold the bullet better, but also reduce barrel fouling. Chamber Of great importance to accuracy is the fit of the ammunition to the barrel. The chamber should be concentric, and the throat sized just slightly larger than the bullet diameter. The cartridge must headspace correctly, be held firmly in place, concentric to the bore, and the bullet must fit the bore and be guided to engage the rifling cleanly. Once the bullet is engaged in the rifling, with a good seal and coaxial fit, it must stay that way. It is often possible to shorten a barrel slightly by removing material from the breech end and re-cut the chamber, which can correct many problems in the original chamber. Crown The crown is the muzzle face of the barrel. The crown's integrity is critical for two reasons: It is the