The golden calibre

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Pre-Dreadnought battleships and some armoured cruisers often had a large calibre secondary armament. The ship designers thought it was not quite practical to add more of the primary calibre (usually 11 or 12 inches) guns in wing turrets, and proceeded to add as powerful guns port and starboard of the centreline as possible.

HMS Agamemnon (c) Emoskopes

 

This practice ended with HMS Dreadnought, and literature usually frames it like this:

The new fire control techniques involved correcting salvo fires, and it took as a rule of thumb at least a salvo of six shells of identical guns to properly observe and thus properly adjust the fall of shot. Similarly large secondary artillery fires or salvoes by other warships on the same target made fall of shot attribution more difficult. Thus the warship designers attempted to have a large-enough calibre for penetrating battleship armour and install this with as big a broadside (6..14 tubes) as affordable on one battleship, which accordingly grew in size.

This is all correct, but a different framing works as well, and explains battleship designs of the Inter-War Years:

Manual loading was quick, reliable and safe only up to a shell weight of about 45 kg, corresponding to six inches calibre (155 mm). Heavier shells required machine-assisted loading, which was slower than manual loading until the 1940's (autoloader 8" guns of Des Moines class cruisers). The 5.5...6" (139...155 mm) calibre range was furthermore considered the minimum to very effectively combat warships of large torpedoboat or destroyer sizes. 4" (100...105 mm) and even about 5" (120...133 mm) were not highly regarded for stopping power against such targets, but were preferred against aircraft.

It made thus little sense to use a bigger secondary artillery calibre than 6" (155 mm).

The calibres between 6" and 8" (such as 17 or 19 cm) largely disappeared after the First World War. They fired almost as slowly as a 12" gun (German ~15" guns of WW2 reached 3.3 rounds per minute).

U.S.American 1930's 6" L/47 gun: 8...10 rounds per minute 

U.S.American 1930's 8" L/55 gun: 3...4 rounds per minute

The optimum, was in the 1920's a 14...15" primary battleship artillery coupled with 5.5...6" secondary artillery (which could be limited to rear 270°, as the best reaction to torpedo-carrying light forces was to go flank speed away from them unless they were detected only very close). 2 pdr pom-poms were the best choice against torpedo bombers and MTBs. The secondary artillery could deal with airships, except for lone cruisers, which might want to have a single 90° elevation 3" (75 mm) gun for that (also as salute gun).

The 28 cm and 33 cm (~13") gun calibres used by new-built ships in Germany and France were kind of budget solutions, suitable for defeating cruisers and unsuitable for battle against battleships. The 8" (~20.3 cm) calibre of the standard heavy cruisers was artificially set by a multinational treaty as limit for cruisers, nobody believed that it was a technical optimum.

The optimum of the 1930's was different, as aircraft had become more threatening. One mix was a divided ~6" secondary artillery with an anti-air 3.5...4" tertiary artillery, but the Americans probably got it right by combining both into their very good 5" L/38 gun.

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I wrote this as an introduction to the concept of a 'golden middle' for artillery calibres.

The 6" calibre was in naval warfare (and in today's army artillery, apparently) the biggest practical calibre before rate of fire fell off very badly, turning guns relatively inefficient against unarmoured or weakly armoured oats and ships.

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Fast forward, and moving to land:

There's again a debate about tank gun calibres. A debate that was postponed in the 1990's, when Germany lengthened its 120 mm smoothbore gun and gave it more powerful propellant, too. The quest for greater penetration power was slowed by a lack of tank protection advances in Eastern Europe and by a focus on improving the projectiles to better deal with late 1980''s protection (heavy ERA). 

There were 140 mm tank guns tested in the Western World during the late 1980's and 1990s, but they were not introduced yet.

prototype of Swiss Leopard 2 version, upgraded with 140 mm gun

 

The German-developed 120 mm smoothbore calibre became a Western de facto standard, while the Soviet legacy 125 mm smoothbore was the competing standard. 125 mm was already too large for manual loading, and was loaded by autoloaders - and in two parts. The separation of ammunition and propellant had a very detrimental effect by the 1980's, when continued development of the arrow projectiles of APFSDS led to longer 'long rod' arrow projectiles than did and do fit into the Soviet-style 125mm shot. The Western munition coped by letting the projectile protrude into the propellant, and thus gained superior penetration despite the smaller calibre.

The Russians moved on in terms of MBT protection with their T-14 Armata tank, so Western developers are again looking at bigger guns.

There's the Rheinmetall 130 mm gun  

There are now again 140 mm tank gun projects:

Reminding myself that the French 140 mm tank gun is an absolute monster and not likely to be pleasant for dismounts in the vicinity looking at the blast coming off that muzzle brake pic.twitter.com/FAkEOqkKgq

— Jon Hawkes (@JonHawkes275) July 30, 2021

The problem with these guns isn't the barrel, which really cannot get significantly longer than the 120 mm L/55 or the 125 mm barrel anyway.

The problem is now the munition. It has gotten unbelievably large to the point that (being of fixed nature, not separate propellant) manual loading is out of question. You need an autoloader for that. Autoloaders are today no more slower than manual loading, but they add a probability of technical failure (~mission self-kill), add cost, add maintenance demands and eliminate a crewmember that participates in security, situational awareness and maintenance efforts. An upside is that the vehicle can be built with a bit lower silhouette with an autoloader.

Moreover, the enormous cartridge size permits for very few cartridges carried, especially if you want to carry the cartridges safely, such as in the turret bustle behind blast doors.

This enormous munition size is really only needed for one thing; a powerful-enough APFSDS shot to penetrate T-14 Armata crew capsule from the frontal 60°.

A 105 mm shell is plenty for just about every other purpose. It may not collapse a family home in one shot, but you can easily shoot twice for the same effect as one 155 mm HE shell would have on it. All armoured vehicles other than MBT frontal protection (would be penetrated by a good 105 mm APFSDS shot. All kinds of 105 mm shots other than smoke and cannister are guaranteed to ruin the turret of a T-14 Armata, achieving a firepower kill.

Is the ability for frontal penetration of the T-14 Armata crew capsule really worth the effort of an even bigger gun than 120 mm L/55?

It sounds like extremism, fanatism and tunnel vision to me. It's regrettable that the German army and arms industry have been especially fixated on APFSDS and frontal 60° tank duels (both concerning firepower and protection).

One should remember that there's an alternative to a bigger gun that can do the very same thing for the tank, from within it: A powerful rocket. The only drawback of the HVM approach is that they need some time for acceleration, thus the ability to penetrate a T-14 Armata hull capsule over the frontal 60° may have a minimum range of a couple hundred metres. You could still mess up its turret at such distances with a gun in the 75...105 mm band, and its sensors with just about anything (including machineguns), but notably with 30 mm autocannon firing AHEAD.

In other words; tanks may have found a golden middle for tank gun calibres decades ago. The candidates are 75...120 mm with single piece munition (75...105 or 90 mm only in combination with missiles). 120 mm smoothbore is the golden middle if one rejects HVMs.

 

related:

The story is very different for stub guns. 

S O

defence_and_freedom@gmx.de

*: Warships smaller than 3,000 tons had at most fragmentation protection armour and early on they also made use of filled coal bunkers to stop fragments. They did not really stop shells.
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