 In 1938 the 155 mm gun T4 on carriage T2 was finally adopted as 155 mm gun M1 on carriage M1.The new gun design used a barrel similar to the earlier 155 mm GPF, but with an Asbury mechanism that incorporated a vertically-hinged breech plug support.This type of breech used an interrupted-thread breech plug with a lock that opened and closed the breech by moving a single lever.