The practice was to move batteries between HAGs as required, and 173rd Siege Bty rapidly transferred to 40th HAG with Fifth Army on 18 October. Fifth Army was engaged in the Battle of the Ancre Heights, the last phase of that summer's Somme Offensive. The fighting ground on despite increasingly bad weather, culminating in the Battle of the Ancre (13–18 November). 173rd Siege Bty remained with Fifth Army during the winter, switching to 10th HAG on 11 December, and back to 40th HAG on 2 February, while Fifth Army carried out a series of small Operations on the Ancre, January–March 1917.