.450
From Halopedia, the Halo wiki
The 11.43mm (.450) cartridge is a non-standard round that was occasionally used in the Sniper Rifle System 99C-S2 AM. It is considerably smaller and lighter than the standard 14.5x114mm round, and ostensibly still capable of dealing considerable damage to a target. It was used by Linda-058 when Blue Team captured Colonel Robert Watts.
This round most likely does not bear any resemblance to the .45 pistol cartridges (including, but not limited to: .45 ACP, .45 LC, and .45 GAP) available in the 20th and 21st century. This is because of the simple fact that pistol cartridges, as opposed to rifle cartridges, are generally not optimized for defeating the standard body armor issued to soldiers of developed nations in the 20th and 21st centuries.
Trivia
- This is not the SRS-99C S2 AM rifle's default chambering. The UNSC's standard cartridge for the weapon is 14.5x114mm, but Linda-058 apparently converted her rifle with an interchangeable barrel assembly in order to use this cartridge.
- ".450" means that this bullet is forty-five hundredths of an inch wide. This is about 11.43mm.
- It should be noted that there was an old .45-caliber rifle cartridge the: .45-70 Government cartridge (colloquially referred to as: ".45 Government"), so a .45-caliber rifle cartridge isn't entirely out of the question. Whether or not such a cartridge would perform effectively when used with the SRS-99C S2 AM is another matter.
