80 years too late - but still an interesting idea (at least to me)

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Back in WW1 the German infantry needed something to take out machine gun nests and pillboxes at a distance. Indirect fires were too inaccurate, bullet fire was unsatisfactory, bazookas didn't exist yet, almost no tanks were in the inventory, flamethrower range was unsatisfactory and light field cannons were too unwieldy. Mountain guns and lightened field cannons were eventually pressed into service as a stop-gap solution.

German crew using a Skoda 75 mm Model 15, 1918

The inter-war years answer for this requirement was the infantry gun (especially the light version). It was a kind of cheaper mountain gun (not meant to be fully disassembled), had a protective shield, lobbed a (slow) 75 mm HE shell (calibres varied internationally; 47 to 76.2 mm calibres were used for the light infantry gun concept; 47 mm was an improvisation) and most importantly: It was crew-mobile.

An infantry gun wasn't portable, but its crew was able to manoeuvre with it on a battlefield, and that satisfied (till enough tanks were available/affordable).

The Brandt system for simple, yet effective infantry mortars was not fully adopted till the early 30's, so these infantry guns were also meant to serve as indirect fire guns of infantry regiments, too. Most missions could actually be met with indirect fire, but infantry gun indirect fire was a bit different than normal mortar or artillery indirect fire. They were usually quite close to the target and their platoon leader was the forward observer - often times there was only a single interruption of the line of sight that protected the crew against spotting and counterfire. A low muzzle velocity and corresponding high trajectory of the shell helped to overshoot this one concealing object. A gun crew close to its forward observer and both being part of the same platoon or company was a huge advantage over normal indirect fire support. Radios were still rare, field telephone cables couldn't be laid over long distance quickly and separating both organisationally would have been an invitation for Clausewitzian friction.

 
The resulting German light infantry gun:

(disregard the fake battle noises)

The First World War spawned another requirement, too: The requirement for a dedicated anti-tank gun. The early tanks were basically slowly moving pillboxes, and posed quite similar problems.

Anti-tank rifles (interesting story) and anti-tank machine guns were unsatisfactory (rifle: not enough effective range and not much effect, machine gun: not enough effective range). A gun, about as light and manoeuvrable as an infantry gun, could meet the anti-tank requirement. It couldn't do so with the slow 75 mm high explosive shell, though. Early (pre-'38) tanks had thin armour, but they became faster. A higher muzzle velocity was desirable in order to actually hit the target at long ranges.

It would have been perfect if the same carriage could be used as for infantry guns, but that was impossible (or was it?): The carriage was only able to absorb a certain amount of recoil, especially at low gun elevation angles.

Recoil is closely related to muzzle energy, and that's weight * muzzle velocity * muzzle velocity; e  = m* v ^ 2, kinetic energy formula.

You can have a relatively heavy high explosive shell, but that restricts muzzle velocity. or you could choose a high muzzle velocity for short time of flight and good accuracy - but that limits the shell weight.

Barrel length created quite the same trade-off dilemma: A long barrel enables a high muzzle velocity, but it would become too heavy if built with a large calibre for suitability for high explosive shells.
75 mm was widely regarded as a good minimum calibre for high explosive shells; anything smaller was considered to be or rather disappointing effect.

The inter-war choice was eventually for 65 to 76.2 mm for light infantry guns (often based on old, suboptimal calibres and guns) and 37 to 47 mm for light anti-tank guns.

There were actually three similar 37 mm calibres in Germany; army AT gun (37 x 249, 192 kJ), Luftwaffe AA gun (37 x 264, 210 kJ) and navy AA gun (37 x 381R, 372 kJ).
The naval gun was least famous, least successful and most powerful in regard to muzzle energy. With hindsight, the naval 37 mm cartridge or the Swedish 40 mm AA gun cartridge would have been better choices (but only marginally better) than the historical 37 mm light AT gun cartridge. Even a standardisation between army AT gun and air force AA gun would have been a good idea.
 

The resulting German light anti-tank gun:

(Again, ignore the fake battle noises.)

Now the idea that keeps fascinating me:

Mountain guns could be assembled and disassembled, even infantry guns had a removable sight. Why not create a convertible gun?

The same carriage could be used for an infantry gun calibre 75 mm and an anti-tank gun calibre 37 mm (naval or at least air force calibre; muzzle brake if necessary). All you needed to do was to remove one gun (barrel+breech+sights) and insert the other one. This exchange could have been quite quick (1-2 min).

Such a gun could have busted above-surface fortifications in direct fire (75 mm HE), provided indirect fire support (75 mm HE, SMK) and defeated lightly armoured tanks (37 mm AP). All this was necessary for infantry regiments (if not battalions!). Many reports about tactical engagements in WW2 mention that 1-2 infantry guns, 1-2 light AT guns, 1-3 mortars were added to company or battalion defensive positions. Their effectiveness was badly needed if infantry had to fight on open terrain.
A convertible gun would have been able to match all three roles; two near-perfectly, one satisfactorily (81.4 mm mortar HE was of course more effective than 75 mm gun shell HE). The net result would have been less logistic troubles, easier positioning and overall greater firepower.

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It would have become outclassed as indirect fire weapon by about the late 30's because of the availability of decent infantry mortars of Brandt system (no more than historical infantry guns were outclassed, though).

It would have become obsolete as anti-tank weapon in 1938-1942 because of increased armour plate strengths (but that could have been partially compensated for with a squeezebore attachment such as the Littlejohn adaptor or a conical barrel - as long as tungsten is available for the penetrator). The historical light AT guns eventually met this fate.

It would have become inferior (but still at least affordable) as infantry direct fire support weapon with the rise of assault guns and CS ("close support") tanks by about 1940. This didn't push the historical light infantry guns out of service, though; they were numerous, but tanks were always in short supply in most armies.

Yet, it would have fared better than the historical light infantry and anti-tank guns:

Its versatility and its ability to exploit shaped charge shells (penetration in 1942 ~ calibre = 75 mm) in the AT role would have kept it more useful. Light AT guns were kept in front-lie service till about '43 and light infantry guns were kept in front-line service till '45, some were even used post-WW2.

Later on, the same carriage could easily have been converted with a smoothbore high-low pressure gun of 75 mm calibre to regain its initial effectiveness in all roles with only one barrel. A 81.4 mm calibre gun with this principle was eventually produced and used with a 50 mm AT gun carriage.

81.4 mm PAW 800 a.k.a. PWK 8H63

The final obsolescence would have been reached sometime in the 1950's when tanks (with their ability to carry a powerful gun) were available for almost all army units and became rather invulnerable to frontal hits with even then-modern 75 mm shaped charge warheads.

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Well, what kept such a convertible gun from being introduced?

I guess there are several reasons. Few armies valued both AT guns and infantry guns highly, and those that did were either in a bad shape during the mid-30's, had an unsuitable doctrine or were in a hurried rearmament.
AT guns were furthermore organised separately from infantry guns; the German AT doctrine preferred highly mobile AT gun batteries, but wartime necessities forced many AT guns into close and quasi-permanent cooperation with infantry units. You cannot protect the infantry against a tank attack with any satisfactory reliability if don't accompany it. The fast-mover AT doctrine was satisfactory against successful tank breakthroughs (especially once it was applied to the tank destroyers instead of towed guns), not so much in regard to preventing said breakthrough.

Light convertible battalion guns would probably have been highly successful (production-wise) if military technology had taken a break in the mid-30's instead of moving forward in great haste.

S O

edit 2013-04:

Same idea, actually attempted soon after WWI, but with too feeble calibres.

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