"The gun system has been retired from US Army service; howitzers above 155 mm caliber are no longer effective as technology has closed the range and firepower gap, and heavier weapon systems require more resources to operate. Gun barrels from retired M110s were initially used as the outer casing in the manufacture of the GBU-28 bunker buster bomb."
Source: http://en.wikipedia.org/wiki/M110_howitzer