Whilst  researching activities in the Nieuport sector in 1917, I came across a graph in the XV Corps Heavy Artillery War Diary for November 1917 (WO-95-927-3 - page 10) recording the serviceability of the guns attached to Corps. Reports and returns are an essential to the logistic system to ensure the availability  of resources to conduct operations. 

The graph covers the period from 22nd June through to 30th November 1917, the period XV Corps occupied the Nieuport sector in preparation for an assault along the Belgian Coast, Operation Hush. (LLT - Operation Hush. A battle that never took place.)

The war diary entry records the Establishment, the number of guns allocated to the Corps Heavy Artillery, and the percentage available.

The graph records a build up to 190 guns by August, reducing to 150 through September, 130 through October, then starts to tail off ahead of XV Corps move to the Ypres Sector. The percentage of guns available ranged between 80% and 65%. The average is 72%, which represents 4 guns out of 6 or 3 out of 4 guns depending on the establishment of a battery. It would total some 50 guns when the establishment was 190 guns. 

The serviceability of Artillery Guns was the subject of a blog post a number of years ago outlining some of the reasons for guns not being available. 

Artillery Guns serviceability - Northumbrian Gunner meanderings - Great War Forum

The ordnance wore and its condition was monitored as a result of firing, barrel life varried howitzers having, in general a longer life than guns, typical values were of the order howitzers 10,000 rounds, guns 7,000 rounds. Recoil systems suffered if not properly maintained; hydraulic buffers leaked and if not replenished allowed excessive recoil causing damage and deterioration of springs. Much of this was repairable by Ordnance workshops. At one stage the workshops of 4th Army were changing 200 sets of springs per day. Carriages could be damaged by enemy fire. Over a 6 week period the workshops of 4th Army repaired some 1,600 field guns and howitzers and declared several hundred beyond repair. Over the period equipment's were firing an average of 2 to 4,000 rounds per equipment.

Old Tom

The methodology when I was serving  was to ensure the number of rounds ordered by the OP would be delivered.  If the order was 5 rounds fire for effect (FFE), then for a six gun battery, 30 round needed to be fired. If only 4 guns were available, to deliver 30 rounds, the order of 5 FFE from the OP, would become 6 or 7 6FFE at the gun end. One wonders if that protocol was in place during WW1. If so the this would put additional  workload on the serviceable guns.

XV Corps Heavy Artillery Serviceability 

22nd June through to 30th November 1917,