What indicates that a round is chambered within the extractor assembly?

The red top edge of the extractor serves as a visual indicator that a round is chambered within the extractor assembly of the M9 Service Pistol. This red color is specifically designed to give the shooter an immediate and unmistakable indication that a cartridge is properly positioned in the chamber. The extractor’s role is to hold the cartridge in place until the firing pin strikes it, and the red coloration helps ensure that the operator can quickly verify the status of the firearm without needing to look into the chamber.

The other options don't provide the same level of clarity regarding the status of a round in the chamber. The color of the ammunition does not indicate whether it is chambered; different types or colors of ammunition can be used interchangeably, resulting in no uniform indicator. The type of magazine used also does not reflect the status of rounds in the chamber, as magazines can be loaded or unloaded independently of the chambered round. Lastly, the position of the slide may indicate if the firearm can be fired, but it does not directly show whether a round is chambered unless combined with other indicators like the extractor.