.450
From Halopedia, the Halo wiki
".450" is the ammunition caliber used by Linda in her sniper rifle when the Spartans captured Colonel Robert Watts.
Trivia
- This is not the sniper rifle's default ammo size. The regular size is 14.5mm, but Linda converted her sniper to use this caliber round.
- ".450" means that this bullet is forty-five hundredths of an inch wide.