The entire WNT might be different, possibly with more tonnage allowance for everyone (which really only benefit Japan, although that's more of any limitations in general benefits Japan as it nullify UK & USA's industrial advantage (well, in the sense of making it less obvious, which actually might have been a bad thing since it gave Japan the delusion that they could actually take on the big boys), and no one cares about the French or Italians besides themselves).With 13 Battleships and Battlecruisers armed with 16 inch guns already in service the Royal Navy does not get the option of Building the Nelson Class in the Washington Treaty. They may lose all of the 13.5 inch gunned ships straight away as well if the total tonnage limit doesn't take into account the increased size of ship need to carry the 16 inch guns.
The 5th Battle Squadron murders Hipper's ships at Jutland. Assuming they can hit the broad side of a barn.
The major problem the RN is going to encounter is that the 15in was built using the same lathes that were used to construct the previous generation of guns, and the 42 calibre barrel length was the largest that could be fitted on those lathes. Basically, if you're going to 16in without some serious concurrent investment at the foundries, you're going to end up with a 39 calibre barrel, which is going to have a low muzzle velocity.
While the 15L42 was one of the most successful guns the RN ever fielded, I'm not sure the Admiralty would be willing to accept that performance penalty.
Practically speaking, rate of fire for battleship guns really doesn’t change. There were some issues with slow firing rates on the Nelson and Colorado classes, but that was a mount issue, not something inherent to the bigger guns, given the Japanese 16” gun and later American guns (including the 16”/50 Mk. 2) did not share the problems. Further proving that point is that the modernized guns for the Colorado’s had the same issues despite being much newer.Yes.
The 15" was capable of penetrating any german ship at the expected engagement range (the poor shell issue would have affected a 16" shell as well, as it would likely just have been a bigger 15" shell)
So no need for 16". That came in at the end of the war, as did the 18", to handle the thickness of armour expected by late/end of war designs.
There is also the issue that, basically, the bigger the shell the slower the rate of fire. You can get more weight of fire on a target with a 15" than a 16".