Which element of a firearm's barrel helps stabilize the bullet?

The lands and grooves in a firearm's barrel are essential for stabilizing the bullet as it travels through the barrel and into the air. When a bullet is fired, the lands (the raised portions of the rifling) engage with the bullet, imparting a spin as it moves down the barrel. This spin is crucial because it stabilizes the bullet in flight, improving accuracy and ensuring a straight trajectory. A well-rifled barrel, which contains the correct pattern of lands and grooves, allows for efficient rotation of the bullet, reducing the chances of it tumbling or deviating from its intended path.

The other options relate to different functions within firearm mechanics but do not directly contribute to the stabilization of the bullet in the same way. A rifled chamber does not exist as part of the typical design; while a muzzle brake serves to reduce recoil, it is not involved in bullet stabilization. Similarly, a counterweight system helps manage balance and control of the firearm but does not affect how the bullet spins during flight. Thus, the lands and grooves are specifically designed to enhance the bullet's flight stability, making them the correct answer for this question.