The last category of expanding bullets is frangible bullets. These are designed to break up on impact, which results in a huge increase in surface area. The most common of these bullets are made of small diameter lead pellets, placed in a thin copper shell and held in place by an epoxy or similar binding agent. On impact, the epoxy shatters and the copper shell opens up, the individual lead balls then spread out in a wide pattern, and due to their low mass to surface area ratio, stop very quickly. Similar bullets are made out of sintered metals, which turn to powder upon impact. These bullets are usually restricted to pistol cartridges and rifle cartridges intended for use at very short ranges, as the nonhomogenous cores tend to cause inaccuracies that, while acceptable at short ranges, are not acceptable for the long ranges at which some rifles are used.