new concept to elevation gearing the counterbalancing means has been described hereinbefore the elevating mechanism is shown in fig7 essentially it consists of a lead screw 139 pivotally fixed at its right hand end to the trunnion support structure 124 and passing through main elevation gearbox 148 near its lefthand end the main elevation gearbox 148 is fast with the cradle 119 119 a the non moving part of the support for barrel 101 a resetter box 147 is pivotted at 147 a to the trunnion support structure 124 and the arrangement is such that the lead screw 139 is parallel to the axis 101 a of the barrel 101 and preferably vertically below it the lead screw 139 passes through a planetary roller screw 149 in the gearbox 148 so that as roller screw 149 rotates the gearbox 148 moves along lead screw 139 in the direction shown by arrows 155 however as both the main elevation gearbox 148 and the resetter gearbox 147 are positively located the result is to cause the barrel 101 and cradle 119 119 a to be elevated or depressed i e angular motion occurs about the horizontal axis through the trunnion bearing 113 and about the pivot 147 a to maintain the axis of barrel 101 a parallel to that of lead screw 139 the means of.