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While some makes of rimfire ratshot cartridges somewhat resemble traditional shotgun shells, with a brass case crimped closed, many other types of rimfire and nearly all centerfire rat-shot cartridges use a hollow plastic capsule, often shaped like a bullet to aid in feeding, which holds the shot. This plastic case shatters during firing, and allows the shot to disperse after it exits the muzzle. There are reports of the plastic casing shattering when being fed from a magazine. Crimped cases do not exhibit this problem but can fail to extract in some semi-automatic guns.
Read more about this topic: Rat-shot