As ordered, the ships were to have six QF Mark XI guns in Mark XX twin mountings in 'A', 'B', and 'X' positions. The 'X' mount gave an estimated arc of fire of 320 degrees at low elevations and 360 degrees at elevations above around 20 degrees. The Mark XI gun itself was a major improvement on the previous version in that it threw a shell (compared to the 50 lb (23 kg) shell in the preceding J class). The Mark XX mount was fully enclosed and supposedly weatherproof; in service, crews found otherwise. It also allowed the guns to be elevated independently. The Mark XX was not technically a turret, as the ammunition feed system was distinct from the weapon mounting, and did not train with the revolving mass. This meant that ammunition supply when the guns were at the limit of training was somewhat difficult. This also meant that the ammunition hoists had to be located between the guns just as in the USN 5" guns. As a result, the axes of the guns were very widely spaced, a feature instantly obvious with the Mark XX mounting.