Alternative History Armoured Fighting Vehicles Part 4

[Bougnas]

As of 1940 the Germans thought that a short 75mm direct-fire gun, i.e. KwK 37 L/24, was the optimal main armament for infantry support AFVs, because smaller-caliber guns' HE shells were much less effective against infantry-battlefield targets. Arguably, the Germans were right. Perhaps the French would have had a similar realization before too much more time had passed.

If the French tank program had continued to evolve, it...much like the early-war German program, with the PzKpfW III and IV, plus the StuG III...might have featured a convergence toward a medium tank armor-powertrain-crewing approach, but a continued divergence of main weapons between tank-fighting and infantry-fighting versions of that otherwise-converged medium tank until the later time when tank-fighting main armament finally evolved to 75mm caliber.

France had at least two short 75mm cannons available...the hull gun from the B1 tank series, and the leftover 75mm Schneider guns from WWI, some of which were in use in casemate versions of the Renault FT. The former probably was much more expensive to build than the KwK 37, so not optimal as a tank weapon. The latter was very old and low-performance. But, either could have provided a starting point for prototypes and battlefield trials.

I suspect that the French were seeing infantry tanks in the same way as the British and the Soviets (T-26 and T-50), with small caliber armament able to take out the odd tank and MG nest. Tougher defenses were to be dealt with by the B series (which were meant to far more common than simple breakthrough/heavy tanks, with more than 60/month in production by mid-1940 and 100/month intended in the future*, same as the Pz III's production rate at the time), or ARL V 39 SPGs. Pz IVs were not produced at very high rates at the time.
It's hard to say. The future battle tank spec had a heavier AT armament than the infantry tanks, so maybe the 47mm SA35 would still be accepted even after facing uparmored German tanks and they would just deal with them using AT guns, tank destroyers and battle tanks. The thing is that by French estimates, a turret suitable for the long 47 is similar in weight to one using the full power 75mm, so if infantry tanks are now required to engage heavier tanks, they will indeed die out in favor of only using medium tanks.

The 75mm hull gun happens to use a cartridge of the same length as the German short 75, but muzzle velocity is higher due to pressure and powder loading being closer to normal guns, so it's probably indeed more expensive to build than your typical infantry/pack howitzer. I suspect the French would also consider it to require a dedicated loader (unlike the 47mm SA35 with tiny light ammo), which would lead to the same weight creep which would kill normal infantry tanks.

So yeah... Either the future infantry tanks still happen in their intended form regardless, or they go the way of the T-50 versus T-34 and are abandonned. This is one bit I did not mention: since the future infantry tanks would no longer use simple truck engines, are heavier and would require 60mm of armor which is more difficult to make than 40mm, their tooling/facility requirements would start approaching those of full-blown medium tanks. They may simply not be spammable enough to be worth it. But we will never know for sure.

 

[Driftless]

This is one bit I did not mention: since the future infantry tanks would no longer use simple truck engines, are heavier and would require 60mm of armor which is more difficult to make than 40mm, their tooling/facility requirements would start approaching those of full-blown medium tanks. They may simply not be spammable enough to be worth it. But we will never know for sure.

Interesting. I hadn't considered there is a leap of scale for the metallurgy of armor manufacture. Sometimes the infrastructure isn't there at the time it's wanted.

Also, I seem to remember that one of the issue for the British 6lb cannon that needed development was the long tool (yeah, yeah, yeah) for boring the barrel - that it was somehow different from other 57mm tools in British industry of the time. Maybe that's a BS idea.

 

[Canuck Wingnut]

Also, I seem to remember that one of the issue for the British 6lb cannon that needed development was the long tool (yeah, yeah, yeah) for boring the barrel - that it was somehow different from other 57mm tools in British industry of the time. Maybe that's a BS idea.

Early marks of the British 6lber were 43 calibers, until tooling for 50 caliber versions was available, while American built 57mms were always 50 caliber- that may be what you're remembering.

 

[JWilly48519]

The British had planned to switch the QF gun factory system over from the two pounder to the six pounder beginning in July-August 1940. Plans and blueprints were prepared, contracts were let, construction and installation engineers and skilled workers were hired and ready, and all the needed equipment was purchased and already on site, ready for installation as soon as the old equipment was removed...except the new, longer barrel bore lathes, which were a very long leadtime item. So initial production was to proceed at the maximum barrel length that would fit on the old lathes.

To compensate for the shorter barrels so that those guns would have the standard muzzle velocity and could be fired with standard sights, and so that all six pounder gunners would have "standard" trajectory and penetration experience when they eventually switched over to standard-length guns, the shorter guns would use different ammo with more propellant.

The plan of course assumed that British and Commonwealth forces would continue to be fully equipped with two pounder towed guns and two pounder equipped tanks, with any needed field replacements during the changeover-shutdown period coming from gun/vehicle inventories in the various theaters of deployment...there being very few towed guns in inventory at the factories, and only enough guns at the tank factories to support continued production operations until it was time to switch over that tank-type.

As the factory system came back into operation after several weeks of shutdown, initial production was to be 100% dedicated to towed guns. As the process of replacement of towed two pounders among all of the first line British and Commonwealth forces began to approach completion, a substantial part of production was to switch over to guns for cruiser tanks. That was anticipated to begin around the end of 1940. Once enough guns for six pounder armed cruiser tanks had been built ahead of the production use-rate to allow that tank production to continue at full speed, new six-pounder-armed infantry tanks were to begin production. That was anticipated to begin around April to June of 1941.

Of course, none of that happened because of the BEF's loss of all equipment shipped to Europe, plus unanticipated losses in North Africa.

 

[sekai]

What would the potential of Romania 75mm/L48 m1943 as a tank gun be? https://en.wikipedia.org/wiki/75_mm_Reșița_Model_1943

According to this source (https://www.deviantart.com/wingsofwrath/art/Resita-75mm-AT-gun-shell-comparison-chart-V5-903141895)
with first hand information, it fires a 6.6kg AP at 840m/s, which is 15% higher muzzle energy than the L48 KwK40. Using 7.2kg AP shell from KwK 42, it should have a muzzle velocity of about 805m/s and similar penetration with KwK 36.

Given it has been installed on the 8.5 ton Maresal tank destroyer, and the ammunition length is only slightly longer than KwK 40. Could it be put on Panzer IV with minimal modification? Would it make continuing production of Panzer IV (perhaps with sloped armor like Ausf.K/L) more attractive than switch to Panther?

 

[Not James Stockdale]

What would the potential of Romania 75mm/L48 m1943 as a tank gun be? https://en.wikipedia.org/wiki/75_mm_Reșița_Model_1943

According to this source (https://www.deviantart.com/wingsofwrath/art/Resita-75mm-AT-gun-shell-comparison-chart-V5-903141895)
with first hand information, it fires a 6.6kg AP at 840m/s, which is 15% higher muzzle energy than the L48 KwK40. Using 7.2kg AP shell from KwK 42, it should have a muzzle velocity of about 805m/s and similar penetration with KwK 36.

Given it has been installed on the 8.5 ton Maresal tank destroyer, and the ammunition length is only slightly longer than KwK 40. Could it be put on Panzer IV with minimal modification? Would it make continuing production of Panzer IV (perhaps with sloped armor like Ausf.K/L) more attractive than switch to Panther?

It might be a good reason to keep the Panther to 35 tons rather than allowing it to grow to 45 tons, but there were other reasons like the Panzer IV drivetrain and suspension to move to a new tank.

 

[Bougnas]

What would the potential of Romania 75mm/L48 m1943 as a tank gun be? https://en.wikipedia.org/wiki/75_mm_Reșița_Model_1943

According to this source (https://www.deviantart.com/wingsofwrath/art/Resita-75mm-AT-gun-shell-comparison-chart-V5-903141895)
with first hand information, it fires a 6.6kg AP at 840m/s, which is 15% higher muzzle energy than the L48 KwK40. Using 7.2kg AP shell from KwK 42, it should have a muzzle velocity of about 805m/s and similar penetration with KwK 36.

Given it has been installed on the 8.5 ton Maresal tank destroyer, and the ammunition length is only slightly longer than KwK 40. Could it be put on Panzer IV with minimal modification? Would it make continuing production of Panzer IV (perhaps with sloped armor like Ausf.K/L) more attractive than switch to Panther?

Well I am not sure what the space claim of the gun itself would be compared to KwK 40 and whether the cartridge can be retained or needs to be necked and shortened, but it might work. I suspect the cartridge as is would be too long since Germany settled on the specific length of the KwK 40 cartridge as opposed to the straight PaK 40 cartridge.

That does not make the tank itself necessarily more attractive though, because as mentionned there was a need for armor and mobility improvements which, considering the chosen layout, were bound to reach 45 tonnes, and even 840 m/s wasn't completely satisfactory for what Germany wanted to do (beyond the need to counter potential enemy armor upgrades, the early ammunition on the KwK 40 had a fairly modest margin of superiority against the upper front armor of a T-34 (improved ammo changed this but Germany wanted to guarantee a safe margin).