It is a neologism, as I have been unable to find a contemporary term for this vital concept. A gun sight, after all, is not related simply to the gun, or the gun and its shell, but to the combination of these two with the firing charge. Again and again you will see that sights have swappable cams for a gun which are shipped or landed when changing from full to reduced charges or from APC to HE shells. In my vernacular, these reconfigurations reflect "a change from one Gun System to another".
- Admiralty, Gunnery Branch. C.B. 1245. Range Tables for His Majesty's Fleet. Volume I. 1918. (9.2-inch Guns and Above.). Pub. No. OU 6089 (late CB 1245), Vol. 1 The National Archives. ADM 186/236.