Which part of a gun barrel is machined to impart spin to the bullet?

The correct answer is the groove, which is an essential component of a gun barrel's rifling. The rifling consists of lands and grooves that are twisted in a spiral pattern inside the barrel. The grooves are specifically designed to engage the bullet as it travels down the barrel.

As the bullet is fired, it fits tightly within the barrel and the grooves grip the bullet's surface, causing it to spin. This spin stabilizes the bullet during flight, making it more accurate and improving its range. This stabilizing effect is crucial because it helps to prevent tumbling and ensures a more predictable trajectory.

In contrast, lands refer to the raised portions of the rifling that also contribute to the bullet's engagement but do not impart the spin directly. The chamber is the part of the gun where the cartridge is loaded and fired, and the grip is simply the part of the firearm that the shooter holds. Therefore, understanding the role of grooves in the context of rifled barrels highlights their importance in firearm accuracy.