Passage: There were thus two main types of heavy, rapid-fire weapons: the manually powered, multiple-barrel machine guns and the single-barrel Maxim guns. By the end of the 19th century, many new designs such as the M1895 Colt–Browning and Hotchkiss were developed, powered by gas operation or recoil operation.  Also, rather than the heavy water jacket, new designs introduced other types of barrel cooling, such as barrel replacement, metal fins, heat sinks or some combination of these.

Question: What different types of barrel cooling are there?
barrel replacement, metal fins, heat sinks