A bit out of order. But let's remember that a lot of artillery calibers were kept around from before WW I to the end of WW II and in some cases till the 2000s.
It is not just the tooling but often there was a stock of hundreds of thousands (millions?) of old shell in storage. The was a great desire to use up at least some of the old shells.
Both the 18 and 25 pdr can get an additional level of usability as the weapon of an SP artillery system. Granted, a suitable doctrine is needed here.
Here we run into several problems. There were actually at least two different 18pdr guns. They used the same ammo (sort of) and things get complicated by several different carriages before we even get into rubber tires. Most of the 18pdrs built during WW I were worn out, destroyed, blown up by the end. The old pole carriage guns should have been retired. The 16 degrees of elevation restricted the range to 5,966 m unless the the crew dug a pit to hold the trail. Range went to 7,100 m. Problems with trying to use these old guns for sp guns is that you can keep the barrel and breech but you need to throw away everything else. 9 degrees of traverse and having to keep a chunk of the old axle was a major problem. So was the meter of recoil when they fired. The later guns (MK III & IV) got variable recoil and higher elevation and the IV had trails that spread and had much greater traverse.
The next problem is that a lot of the MK IIIs and IVs were re-tubed/barreled to the early 25pdrs and there weren't very many 'modern' 18pdrs left and after Dunkirk there were almost none. Pre-Dunkirk there weren't enough 25pdrs for the 10 divisions and some divisions had two regiments of 18pdrs or 18/25s and one regiment of the 4.5in howitzer.
From Wiki
"
A total of 216 guns were lost in the 1940 campaign. This left the British Army with 126 guns in UK and 130 in the rest of the world, according to a stocktake in July 1940."
Any plans to arm tracked chassis should take this into account.
In typical British fashion the 4.5in howitzer only shared the bore size with 4.5 in gun and the ammo was definitely not interchangeable in anyway. This 1913-14 weapon was sturdy and well liked but it had rather short range. The Shell weight was only 15.7kg and it was planned to replace them with the 25pdr gun as soon as possible.
To me, it seems like the British were keen to switch the calibres across the board, leaving a lot of overlap in the process. Combine it with the late start with the modernization program of artillery, as well as the long drawn-out introduction of the 'proper' 25 pounder, and their artillery park of 1940 looked a lot like that of 1918.
Mostly covered above for the small guns.
I'd try and get better mileage from the 60 pdr before embarking on something bigger, and forget the 4.5in cannon.
Well, there were two distinctly different 60pdr although they could share ammo, mostly. The older 60pdr (1904-5) had limited elevation and limited traverse and had a pair of recoil cylinder on top of the barrel. The 1918 gun was pretty much brand new but would fire the old shells. But since these were peace time guns (war had ended before production geared up) there were not all that many and since 76 (all the carriages that were deemed suitable) of them were converted with new barrels to form the 4.5in MK I it would seem the British were trying to get more mileage out of them.
The old 60pdr WW I shells were blunt and had poor ballistics. They did make some shells with somewhat better streamlining but the 4.5in shells were much better and account for quite of bit of the range increase.
The 4.5in gun was the only really long range gun the British had. 18,750-19,200 meters But the crappy shell hindered it's effectiveness.
As a next step, I don't see the niche for opting for an all-new caliber that 5.5 in was, so I'd suggest simply continuing with the 6in route, but with a new gun, that can again be reasonably light, while offering the range in-between the 6in howitzer and cannon, or perhaps tad better than the 15cm sFh18.
The new 6in gun and the 60 pdr cannon might have shared the carriage.
I am not sure quite what the thinking was behind the 5.5 in except that the 5.5in could fire a 100lb/44.5kg shell at 1675fps for 16,200yds range and the 4.5 in barrel would fit on the same carriage and recoil system for a 55lb shell at 2250fps and 20,500yds range. They even used the same breech mechanism.
The 4.5in only had a HE shell. No smoke or other specialty ammunition.
When the British finally woke up (1943) and designed a light 5.5in shell with better steel and a more streamline shape the 5.5in gun hit 1950fps and 18,100yds range.
To show the importance of artillery the British 5.5in guns of 21st army group in NW Europe fire 2,610,747 rounds between D-Day and VE Day. using 80lbs (and many were not) per round that is 104,429 tons of 5.5in artillery ammunition fired. Or about 17,400 4 engine bomber loads at 6 tons per load.
The British 21st Army also fired 159,898 rounds of 7.2in howitzer shells (200lbs each) 15,989 tons.
I don't have the figures for the smaller guns or the Lend Lease guns.
BTW the 5.5in barrel/breech weighed 4120lbs, the old 6in WW I gun barrel/breech weighed 10,248lbs. The US 155mm M1 howitzer barrel/breech weighed 3,825lbs, the American 155mm M1 gun (Long Tom) barrel/breech was 9595lbs. This was the gun that could fire a 95lb shell 25,395 yds. But the weight in action was 30,600lbs. Powerful, long range 6in guns were heavy and required special tractors/tow Vehicles.
Despite the huge importance of it.
