The Lord Nelson-class battleships were designed and built at a time when the direction of future battleship construction was controversial. On the one hand, naval combat during the Russo–Japanese War of 1904–1905 suggested that engagement ranges would increase to the point that intermediate and secondary batteries would become far less important and perhaps even ineffective, and that smaller-calibre guns would be useless in combat between capital ships; on the other hand, the lower rate of fire of battleship main batteries raised questions about the prudence of building all-big-gun battleships, for fear that they might be overwhelmed by the higher rate of fire of intermediate-calibre guns in the shorter-range engagements that might occur in fog or bad weather or at night. In the end, the all-big-gun battleships, which became known as dreadnoughts after the first such ship, Dreadnought, were vindicated, but this was by no means clear when the Lord Nelsons were designed in 1904 or even by the time they were laid down in 1905.
The Lord Nelsons were the first battleships for which Sir Phillip Watts was responsible. Although they followed the pre-dreadnought pattern established in the Royal Sovereign-class battleships in the early 1890s of having two twin main battery mounts, one fore and one aft, and mounted a main battery of four 12 inch (305 mm) guns, as had every pre-dreadnought since those of the Majestic-class battleships in the mid-1890s, they otherwise were a major departure from previous British pre-dreadnought designs; they might have marked a new era in pre-dreadnought design had not the rise of the dreadnoughts snuffed out the pre-dreadnought era. In order to match increases in firepower seen in foreign battleships of similar displacement, the preceding King Edward VII-class battleships had introduced a 9.2-inch (234 mm) intermediate battery into British battleships in addition to the 6 inch (152 mm) secondary battery they long had mounted, but the Lord Nelsons carried this further by mounting an all-9.2-inch secondary battery; they were the first British battleships not to mount 6 inch guns since HMS Inflexible, which joined the fleet in 1881. (The Trafalgar and Centurion classes had joined the fleet with 4.7 inch (120 mm) secondaries but had later had them replaced by 6 inch guns.) Also, the 9.2-inch battery, made up of more powerful guns than on the King Edward VII-class ships, was mounted in turrets (four double and two single) on the upper deck, rather than on the main deck in a central battery or casemates; this eliminated the problem of being unable to work the secondaries in a seaway, a problem in the many classes of British battleships with main-deck-mounted secondaries which were washed out in all but the calmest weather.
The 12 inch guns were a new, more powerful, 45-calibre type. They and their turrets were the same as those carried by the revolutionary Dreadnought. Indeed, the completion of Lord Nelson and Agamemnon was delayed when their main battery guns and mountings were diverted to Dreadnought to expedite her completion in 1906.
In the end, the mixed-calibre heavy armament proved unsuccessful, as gunnery officers found it impossible to distinguish between 12-inch (305-mm) and 9.2-inch shell splashes, making fire control impractical. This finding further pushed the navies of the world to move to all-big-gun dreadnought battleship designs. Indeed, an all-big-gun design had been considered for the Lord Nelsons in January 1905, but their design was too far advanced by then to be changed, and the all-big-gun layout had to await HMS Dreadnought.
For anti-torpedo-boat defense, the Lord Nelsons retained a battery of 12-pounders. These were mounted on a large flying deck amidships, where they had a good field of fire. However, this innovative mounting scheme also was criticised because it made a good target and because in combat falling debris due to damage might foul the 9.2-inch turrets below. In addition, some officers believed that the all-12-pounder battery was too light to deal with larger, modern torpedo boats.