This is a sound moderator aka “silencer” for an airgun or paintball gun. Pump-up airguns in particular make a lot of noise as the air rushes out, and the moderator cotains a series of what are called “K Baffles” (see cutaway one in the photo) that slow it down so that it does not make a sharp >Crak!< The pellet goes unimpeeded down the middle, and may even slightly gain velocity. This design is remarkably efficient.
Supersonic projectiles will always go crack anyway and a silencer is pretty pointless with them. Firearms are likely to blow the silencer to bits.
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