.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 sniper rifle's default ammo size. The regular size is 14.5mm, but Linda converted her sniper with an interchangeable barrel to use this caliber round.
 * ".450" means that this bullet is forty-five hundredths of an inch wide. This is about 11.43mm.