The major limitation in higher rates of fire arises due to the problem of heat. Even a manually operated rifle generates heat as rounds are fired. A machine gun builds up heat so rapidly that steps must be taken to prevent overheating. Solutions include making barrels heavier so that they heat up more slowly, making barrels rapidly replaceable by the crews, or using water jackets around the barrel to cool the weapon. A modern machine gun team will carry at least one spare barrel for their weapon, which can be swapped out within a few seconds by a trained crew. Problems with overheating can range from ammunition firing unintentionally (cook-off), or, what is much worse in combat, failure to fire or explosion of the weapon.