.450

.450 is a non-standard ammunition caliber used in the Sniper Rifle System 99C-S2 AM. It is considerably smaller than the standard 14.5mm rounds, but are lighter, and still capable of dealing considerable damage to a target. It was used by SPARTAN-058 (Linda) when Blue Team captured Colonel Robert Watts.

This round most likely does not bear any resemblance to the .45 rounds used by 20th and 21st century armies. This is because most .45 rounds in the 21st century are for handguns and thus do not contain enough powder for use as a sniper round. The .45 ACP, a common 21st century round, is poor at penetrating 21st century armor.

Trivia

 * This is not the SRS-99C S2 AM rifle's default ammo size. The regular size is 14.5mm, but Linda converted her rifle with an interchangeable barrel and chamber 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 is another story.