What can result from residue or rust in the barrel?

Residue or rust in the barrel can create serious pressure build-up during firing. When a bullet is fired, it requires a clear path to exit the barrel. Any obstruction, such as residue or rust, can block this path, leading to a confinement of gases that results in increased pressure within the barrel. This excessive pressure can potentially cause dangerous situations, such as barrel rupture or malfunctioning of the firearm, which poses a significant safety risk to the shooter and bystanders.

Reduced accuracy can occur due to external factors affecting the bullet's flight path, but it is not as immediately dangerous as pressure build-up. Increased recoil is typically a product of the firearm's mechanics and ammunition type rather than residue or rust in the barrel. Firing pin failure relates to the mechanism that ignites the cartridge and is generally unrelated to the condition of the barrel. Therefore, the seriousness of pressure build-up due to residue or rust clearly underscores why it's crucial to maintain the interior condition of the barrel.