This will be about the biggest naval gun ever. Eventually.
Well first I was looking up the etymology of "monitor", which as
I'm sure you'll agree is a very normal thing to do, Specifically I was
wondering about the connection with warships, which sure enough comes
from USS Monitor, which was such a success that the general pattern
of ship that followed (big gun in a turret, armour) was known as
"monitors". OK, that's fair enough.
But Wiktionary, specifically: "13 (archaic) an ironclad."
Well, no, not quite, at least in my idiolect. "Monitor" in the sense
of a warship means a little more than that to me, it means a very slow
ship, often heavily armoured, intended for bombarding shore targets.
It turns out that this is a later British usage, more or less early
20th century. (I asked a Canadian naval historian, but to him it's
whichever of the two separate uses people are already talking about.)
And that led me to the Lord Clive class of monitor, originally built
with twin 12" turrets taken off decommissioned Majestic battleships.
The gun plus turret is effectively a module that can be shifted about,
assuming the availability of huge dockyard cranes; apart from anything
else, there are many things that can put a warship's turret out of
service but keep the warship otherwise usable, and it's useful to be
able to drop in a spare while the other one is repaired. But then
something else happened.
Of course it's all Jackie Fisher's fault. A great man in many
respects, but he was unreasonably enthusiastic for the battlecruiser
(i.e. battleship guns on a faster and more lightly armoured hull).
Being prevented from doing this to the extremes he wanted, he proposed
a class of "large light cruisers" (I am inevitably reminded of the
"through-deck cruisers", i.e. aircraft carriers, of the 1970s) which
became the Courageous class, with four 15-inch guns. But a thing
worth doing is worth overdoing, and Furious was modified to take two
18" guns instead.
(Which isn't a great idea for gunnery. When you fire a salvo of say
twelve shells, all your guns are aimed at basically the same point
(I'm over-simplifying); the shells spread out and land in an area,
which you observe, and you use the centre of that area to correct your
gun-laying settings. But if you only have one or two shells in a
salvo, you don't know what is systematic error in laying the guns and
what is random error for an individual shell.)
This took a while. The gun was an enlarged version of the 15; Elswick
Ordnance started to design it in 1915, and it was tried out in
Furious in 1917. And it was simply Too Much; remember that these
ships are functionally battle cruisers, not battle ships, and
Furious didn't have the armour and sheer mass to stand up to firing
such a huge gun. Also she was being hastily converted to an
aircraft carrier, though flight decks hadn't quite been worked out
yet. But that's another story.
The 18" guns were not needed, but two of them had been fitted to
Furious and a third was available as a spare. So we come back to the
Lord Clive class monitor. Two of these, Lord Clive and General
Wolfe, were modified. Not to swap the twin-12 turret for the 18, of
course, that would be ridiculous. No, the original turret was left in
place, and the new turret was mounted on the rear deck. Well, turret
is putting it too string. It was a fairly fixed mount, with the gun
pointing permanently to starboard and with a potential traverse of
about ten degrees (any more than that, move the ship). It could
elevate to 45° or down to 22° (anything lower would have needed more
bracing, of which there was quite a lot already), but had to be loaded
at 10. Operation was by a combination of hydraulics and muscle.
General Wolfe began firing trials on 7 August 1918. On 28 September
the gun was fired in earnest at the Snaeskerke railway bridge from
36,000 yards away, probably the longest range shot by a naval gun in
action ever; the recoil not only made the ship roll but shifted her
sideways against her anchors, which cut the rate of fire
substantially. Lord Clive was back in service with the new gun on 13
October, and fired a few shots. The third gun was intended for Prince
Eugene but was not fitted before the end of the war.
Is this the biggest naval gun? Depends on how you count it. The 46
cm/45 Type 94 guns used on Yamato and Musashi twenty years later
were of slightly greater calibre, by 2.8mm, but fired a lighter shell;
they were technically capable of engaging at a longer range, but never
did so. (And the guns on the British N3 battleships would have been
similar, but they were never built.)