1. There are adapters commercially available that fit inside a gunbarrel like an arm inside a sleeve, so the weapon can fire a smaller caliber of ammo than it would normally take. Some of these subcaliber inserts are rifled, and some are not. If it is, then the recovered bullets will carry the rifling of the adapter, not the weapon. If it’s not, the bullet will have no rifling marks at all.
2. Using smaller caliber ammo without an adapter might also leave no rifling marks on the bullet. Of course, the bullet may not pack much punch, either.
3. Frangible bullets, which are designed to shatter upon striking a hard surface to prevent riccochets, and exploding bullets, which are exactly what they sound like, don’t leave a lot behind for forensic examination.
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