I wish we lived in a world where firearms could be dispensed with. Sadly, even the most civilised of societies rely on weapons (albeit tacitly) for their continuance.
Today’s invention attempts to ensure that any bullets that get fired do so more accurately and that every one is linked to the shell casing (as well as the barrel) from which it emerged.
In addition to using rifling in the barrel of a firearm to stabilise it in flight, new bullets would have a projectile with a thin-walled cylindrical tail section, extending back into the body of the shell casing and in contact with it. The casing itself would be rifled internally; effectively adding barrel length to the weapon in question as well as somewhat increased spin/stability. As the cylindrical tail moves out of the casing (locked in the weapon), it spins, perhaps 20% earlier than usual, due to the casing rifling with which it engages.
Rifling marks on the bullet would match the grooves on the casing interior (these could be given a very slight variation in geometry, unique to a given casing and would be hard to tamper with). Even if the gun is removed from a crimescene, people engaged in illegal activity will find it difficult to gather all their expended casings.