Frangible bullets are intended to disintegrate into tiny particles upon target impact to minimize their penetration of other objects. Small particles are slowed more rapidly by air resistance, and are less likely to cause injury or damage to persons and objects distant from the point of bullet impact. Most frangible bullets are subject to brittle failure upon striking a hard target. This mechanism has been used to minimize the tendency of malleable lead and copper bullets to ricochet from hard targets as large, cohesive particles. Brittle failure may occur at subsonic velocity. Attempting to crimp a brittle frangible bullet into the cartridge case may break the bullet. Brittle frangible bullets may break during the self-loading cycle of semi-automatic firearms; and those fired from revolvers often break as the bullet encounters the barrel forcing cone after leaving the cylinder. Manufacturing Powder metallurgy techniques fabricate bullets from mixtures of powdered metals (typically tin, copper, zinc, and/or tungsten) compressed at room temperature to produce a high-density material. Mechanical interlocking and cold welding bond the metals together either pressed directly to shape, or into bar stock billets that can be swaged into projectiles, with or without jacketing. Alternative manufacturing techniques include heat treating or sintering powdered metals at temperatures below the melting point, or binding the powdered metal with an adhesive or polymer in an injection moulding. Target damage The mechanism of bullet disintegration varies with the energy transfer at the time of impact. With sufficient velocity, bullets may be vaporized upon impact. Few firearms can propel bullets at sufficient velocity to cause reliable vaporization at the target, and air resistance causes bullet velocity to decrease with increasing distance from the firing point; so frangible bullets typically rely upon other mechanisms for disintegration at lower velocities. Target characteristics are an important aspect of interaction with the bullet. Energy available to initiate the disintegration mechanism is limited by the rate at which the target slows the bullet; so bullets may pass through flexible, fragile or low-density materials without slowing the bullet enough to cause disintegration. Bullets must resist disintegration during handling, loading, and firing to reliably hit the target; so high-velocity loads may require a non-frangible jacket to protect a frangible core from disintegration prior to target impact. The jacket may ricochet, but should have reduced range without the weight of the frangible core. Frangible hollow-point bullets may penetrate clothing, drywall, and light sheet metal;