Barrel material can refer to a variety of materials used in the construction of barrels, such as wood, metal, and plastic. Two common types of wood used for barrel making are OAK and ELM, both of which have three letters. OAK is a hardwood that is known for its strength and durability, making it an ideal choice for barrel making. ELM is also a hardwood, but it is more flexible than OAK, allowing it to be bent into curved shapes. Both woods are popular choices for barrel making due to their strength and durability.