Abstract: A first member, such as a primer cartridge (33), is inserted in an aperture in a second member, such as a chamber (8) in a gun breech block (4,5), by means of an apparatus such as a primer loader (52), comprising a tubular member whose bore (16) serves as a guide for positioning the first member and which is aligned with the aperture in the second member. A fourth member, such as a ramming rod (19), may be provided to move the first member into the guide and thence into the aperture as the fourth member advances from a first to a second position. In the case where the primer cartridge passes across a gap (18) in order to enter the aperture, the fourth member may be arranged to advance an additional distance to a third position wherein the additional distance is not less than the width of the gap, the additional distance being the permitted movement of a fifth member, such as a slidable bush (38), between two positions.