The U.S. Army has started diversifying its supplier base for 155mm artillery shells, moving away from the bottleneck of a single source that has endangered the flow of fresh ammo, according to a top service official.The service is racing toward a goal of shoring up all major single sources that provide parts or materials for 155mm munitions by the end of 2025.“There’s going to be a lot of ribbon cuttings between now and the end of the year,” Dou…