Hello, Matt. According to Stanton's Order of Battle, the 18th Field Artillery Battalion was a nondivisional unit. That means that they would not have been a permanent part of a division, but would have been attached to larger units, from time to time, as needed. They were a 105mm Howitzer Truck Drawn outfit. They were formed 9 Aug 43 at Camp Gordon, GA from elements of the 685th Field Artillery Battalion. They left NYNY on 11 Feb 44, arrived England 23 Feb 44, and France (ETO) 6 July 44. They got campaign credit for Normandy, Northern France, Ardennes-Alsace, Central Europe, and Rhineland.
I'm sure 844 was the numerical designation for his military occupational specialty (MOS) - Gun Crewman, Light Artillery. In other words, one of the men who operated the 105MM Howitzer.