The statement that standard bullets can be manually modified into expanding or fragmenting bullets (commonly called dum-dum bullets) using basic tools like a knife is a matter of everyday mechanical fact and historical practice rather than formal academic study, requiring no citation.
The claim refers to the physical alteration of ammunition (filing, cutting, or notching the tip of a full metal jacket bullet to cause expansion upon impact). While forensic literature discusses terminal ballistics, expanding bullets, and prefragmented projectiles, manual modification of ammunition is a well-known historical and practical phenomenon (dum-dum bullets) rather than a specialized scientific hypothesis tested in peer-reviewed literature. Therefore, it falls under common knowledge/general mechanics, and none of the retrieved technical papers directly test or study the act of manually modifying bullets with a knife.