Artillery calibres and concepts

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The most common artillery (gun) calibres are nowadays in the 105...155 mm range. Bigger guns largely fell out of favour due to aircraft bombs taking over their roles and smaller guns largely fell out of favour because direct artillery fires lost importance post-1914.

 

75 mm was considered the minimum calibre for a high explosive (HE) round to be really good at supporting infantry with direct fires. Infantry support tanks of late 1930's received guns of such a calibre, albeit wartime experiences led to the introduction of bigger calibres (up to 152 mm plus one freak design) for that role.

75 mm infantry gun, for supporting infantry

75 mm calibre guns had often a high muzzle velocity, which gave them a secondary anti-tank capability. This also allowed for a little better ranges than the common 105 mm howitzers of the time had. Their indirect fires usefulness was hampered by the small explosion signature (bang, smoke, dust) of their 75 mm HE shells, though. It was difficult to observe the explosions with sound ranging or visual observation from afar. There's no correction of fires without feedback from observers, thus poor accuracy.

105 mm howitzer HE was an indirect fires sweet spot for a long time, and probably still is. The Russian/Soviet 122 mm calibre was a bit more powerful and the British 87.6 mm (25 pdr) calibre was fairly close as well, but 105 mm was understood to be very efficient for fragmentation effect and suppression. 150...155 mm HE shells were more powerful individually, but covered less area with fragmentation effect or suppression effect than did the 105 mm HE - pound for pound. 105 mm HE also enjoyed a reputation for being more capable at shooting danger close (at targets very close to friendlies), albeit this is a negligible difference when the guns don't have a tight dispersion.

I've seen tables and lessons learned about 105 and 150 mm howitzers use in WW2 and the conclusion from the different quantities of munitions needed for certain effects was simple: You did not need more 105 mm munitions mass for the purpose than 150 mm munitions mass. The calibres were largely substitutes for effects. You needed more 105 mm shells for the job than 150 mm shells, but a 105 mm shell weighs about 17 kg, and a 150 mm shell weighed about 40 kg. 105 mm howitzers also had a little higher burst rate of fire (fires in first minute before the barrel is hot and rate of fire has to match how quickly the barrel cools down). 150 mm howitzers had some manpower efficiency and sustained firepower advantages (both calibres sustained about the same rate of fire).

150...155 mm had two big advantages over 105 mm  during the world wars, though. One, it was more capable of piercing overhead cover and the roofs of field fortification bunkers (several layers of wooden logs plus earth, supported by wooden logs). This is where the 150...155 mm success came from; Germans tested field fortifications around 1900 and came to the conclusion that a 150 mm shell was required to penetrate a field bunker (but keep in mind howitzers had poor muzzle velocities at that time). Such intent to destroy almost entire sections or platoons in a field bunker led to a mass employment of heavy howitzers (and even heavier artillery) in the Great War a.k.a. First World War.

Two, larger calibres are also more efficient for long range (better ratio of kinetic energy to drag), and howitzers grew into high velocity (and thus long range) guns especially after the Cold War. Today's 155 mm L/52 howitzers have ballistics that rival late WW2 high-performance cannons of about the same calibre.

modern 155 mm L/52:  43...43.5 kg shell, approx. 945 m/s with maximum charges

150 mm Type 5 (1945): 41 kg shell, 930 m/s

155 mm is thus superior to 105...122 mm for two purposes; destructive fires on field fortifications and long range fires. The latter were a nice niche for dumb shells in the 1990's till 00's, but it's obvious that guided rockets are more suitable for it because it's much easier to give guidance to a rocket than to an abruptly launched (electronics need be hardened to this shock) projectile. Shells are normally spin-stabilised, which prohibits normal steering by rudders. Guided 155 mm projectiles are fin-stabilised and thus differ only in the mode of acceleration from rockets. The barrel restricts the calibre, so using rockets launched from pods makes much more sense for long range fires.

Which leaves us with the destructive fires on field fortifications (and rare effects on armoured vehicles) for the 155 m calibre. So let's talk about what I meant with "destructive fires".

There are harassing fires, suppressive fires, neutralising fires and destructive fires.

Harassing fires are almost always bollocks, waste of munitions and at times plain war crimes (if shot at the wrong coordinates).

Suppressive fires are more intense and shall limit hostile activities, especially how much hostiles dare to move or shoot. It's not so effective at limiting how much they observe and report. Suppressive fires may make sense to support (= protect) exposed (usually that's = moving) friendly forces.

Neutralising fires are more intense than suppressive fires and shall render hostiles combat ineffective through psychological effects (up to shell shock). The targeted troops may have a few per cent losses of personnel and material through blast or fragments, but that's not what matters. Properly done neutralising fires allow almost none of the targeted hostiles to be effective shooters for minutes after the end of the neutralising fires. Neutralising fires make much sense when friendly troops assault and take the targeted position soon after the neutralising fires ceased. Neutralising fires are thus most important for manoeuvring forces and for attacks with limited objectives (by forces of the line).

Destructive fires are not necessarily more intense, but they require much more munitions (or precise knowledge of hostile positions and great precision at firing at them with powerful-enough munitions). This is wasteful for manoeuvring forces such as a armoured formation on a rampage after breaking through a frontline. Destructive fires usually take more time and more munitions than neutralising fires (and may make the ground less trafficable by cratering), which is really bad for a manoeuvre force (force of exploitation).

also see FM 3-09 pages 3-25 and 3-26 if in doubt

Coming back to the forces of the line and forces of exploitation distinction, we can see that the middle calibre group of 100...105 (maybe up to 122) mm calibre was very suitable to support forces of exploitation.

The same calibre group was also very suitable for forces of the line most of the time. The two exceptions were long range fires (especially in WWI), which could be left to levels above divisions and destructive fires. Bigger calibres for destructive fires were not essential when breakthrough (using artillery mostly for neutralisation of the assaulted positions and suppression of other possibly intervening positions) was possible. The possibility of breakthrough depends more on morale and competence than on equipment, and some armies had this competence from late 1917 onwards.

The actual niche for 150...155 mm calibres was thus attrition through destructive fires along the frontline. This was likely the main reason for why 150...155 mm "heavy" howitzers were used by most European and American infantry divisions of WW2 and also considered essential during WW1. Their popularity also stemmed a bit from (slightly) better ranges than 105 mm had and subjective preference of bigger bangs. The weights particularly of WW2 howitzers were troublesome if not intolerable for horse draft, though.

152...155 mm calibre munitions are also more efficient for the deployment of cluster submunition types than 105 mm, but this advantage is now gone due to the widely followed cluster munitions ban.

The smaller size of 105 mm guns and their easy integration into wheeled motor vehicles (such as 6x6 or 8x8 lorries, which allows for a fragmentation and bullet protection, 360° turret, 85° maximum elevation, whole crew onboard, RCWS and lots of munitions onboard) as well as their danger close qualities position them as rivals of 120 mm mortars. Meanwhile, their muzzle velocity allows their use as heavy anti-air gun against easy targets (such as multicopters) with appropriate proximity or electronic time fuses. Their much better range (and dispersion at range) than 120 mm mortars opens up opportunities for covering larger areas, including assisting neighbouring units by massing fires.

 

So long story short; 105 mm is still a great calibre. 155 mm is more impressive and can do almost all the same jobs (except danger close shots), but 155 mm is far from a necessity. A mix of 105 mm howitzers and precision-guided rockets of long range (as much maximum range as target detection range) would do the artillery job just fine and render mortars unnecessary. 105 mm is less demanding regarding platforms, purchase costs and just generally lends itself better to the battalion battlegroup organic fires role (and to small brigades).

 

related:

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