Bullet

A bullet was a projectile propelled by a firearm. Most bullets were made from a type of metal. Sam Swift was of the view that "no brain outwits a bullet". (TV: The Woman Who Lived) Similarily, Graham O'Brien believed that one could not "out-think bullets". The Thirteenth Doctor, however, noted that she had done so all her life, stating "brains beat bullets." (TV: The Ghost Monument)

Effect and defense
In human mythology, bullets made of silver could be used to kill a werewolf. A "real" werewolf like the Lupine Wavelength Haemovariform was unharmed by ordinary bullets; as the Tenth Doctor said to Captain Reynolds, "Bullets can't stop it." (TV: Tooth and Claw) Silver bullets were also utilised by the Brigadier to defeat the Destroyer. UNIT also used teflon anti-Dalek bullets and gold-tipped bullets for use against Cybermen. (TV: Battlefield)

The Cybermen of Pete's World proved invulnerable to bullets, (TV: Rise of the Cybermen, Doomsday) as did survival armour used by Ice Warriors. (TV: The Seeds of Death, Empress of Mars)

The Experimental Prototype Robot K1 was immune to bullets much to the frustration of the Brigadier, who commented "just once I'd like to meet an alien menace that wasn't immune to bullets." (TV: Robot)

In the Sontaran invasion of Earth in 2009, the Sontarans used a cordolaine signal excited the copper atoms in bullets and caused their expansion within a gun barrel, rendering Earth-made guns useless. The Doctor admonished General Staal for this, stating that "a Sontaran should face bullets with dignity". UNIT managed a counter-measure by using rad-steel coated bullets. This made the cordolaine signal ineffective and the Sontarans, despite their use of Sontaran space armour, were easily defeated by UNIT forces. (TV: The Sontaran Stratagem/The Poison Sky)

The Ghost's superhero suit withstood bullets fired by Sim. (TV: The Return of Doctor Mysterio)

Bastic bullets, bullets fitted with bastic heads, were used to destroy pre-war Daleks, (TV: Revelation of the Daleks) but eventually Dalek force field technology advanced to the point where bastic bullets were no longer effective against them, though the eyepiece remained susceptible. (TV: The Parting of the Ways)

The matriarch of the Klep-Clotch Slitheen boasted that bullets would bounce off her Raxacoricofallapatorian hide. (AUDIO: Death on the Mile)

Though they could be temporarily incapacitated by gunshots, Weevils were resistant to bullets and quickly recovered. (TV: Exit Wounds)

Usage
Gustave Lytton and his men, who worked on behalf of the Dalek Task Force, used bullets while operating on 20th century Earth. Having been recently fired, some of these bullets were found on a warehouse floor by Tegan Jovanka, who observed that they were "hardly alien". The Fifth Doctor, however, rhetorically asked "why advertise who you are?" (TV: Resurrection of the Daleks)

Similarily, Matron Cofelia's guards used guns with bullets. (TV: Partners in Crime)

The Seventh Doctor sustained three gunshot wounds at the hands of a gang in San Francisco on 30 December 1999; one bullet went straight through his shoulder, inflicting no damage, while the other two were caught in his left leg before being removed at Walker General Hospital. (TV: Doctor Who)

Immediately following the Year That Never Was, Lucy Saxon inflicted a gunshot wound against. Though the Tenth Doctor noted that the Master, as a Time Lord, could easily survive "one little bullet" by regenerating, the Master purposefully chose not to and so died. (TV: Last of the Time Lords)

During the 21st century Dalek invasion, the presence of the time-lock within the Hub became apparent when the bullets fired by Gwen Cooper and Ianto Jones against a Dalek were suspended in mid-air. (TV: Journey's End)

Expression
In a shared dream induced by the Dream Lord, the Eleventh Doctor covered Amy Pond and Rory Williams' escape as he confronted the Eknodine, endangering himself. Following this, Amy noted to Rory that the Doctor had "[taken] the bullet" for them. (TV: Amy's Choice)

The Thirteenth Doctor cited the peaceful resolution to the Cuban Missile Crisis as the Earth dodging a "really big bullet". (COMIC: The Piggybackers)