The 28/20 guns, just under 2800 built and it was small enough to be useful for troops without motor transport, it was also small enough to be easily mounted in small trucks/heavy cars and in small armored vehicles. The Sd. Kfz 222 armored car being one and the small 1/2 track another. Hard to say if more motor transport would have ben more useful. It gave some needed punch to the parachute and mountain troops.
One wonders how much the 28/20 was actually a good gun to go with anyway? While the set amount of tungsten will last longer here vs. the bigger guns, the actual effect - tank killed - would've probably remained the same if they stuck with the 37mm instead. For the instances where the light weight is a paramount, and as-is the 37 is still too heavy - install the muzzle brakes and perhaps it can fit on the French 25mm carriages, and call it a day?
The 42/28mm guns, about 313 built, mounted on modified 37mm Pak 36 carriages. Some went to the "light" troops. Others were spread around. AP had very good penetration for the size of the gun. Effects on things behind the armor was a little lacking. Ability to fire effective HE is rather lacking, HE projectile weighed 280 grams. A few may have been mounted in 1/2 tracks or on tracked carriers (Bren or French?).
This one would've probably been a good retrofit on the Pz-IIIs that came out from production as armed with the 3.7cm gun? Or even probably for the Pz-38(t). At 500 and 1000m, the penetration is more than double that of the 3.7cm with APCR, or in the ballpark with the short 5cm, that was a gun the turret on the 38(t) will not be accepting happily, if at all.
The 75/55mm guns. only around 150 built. Motorizing them may have been more effective. But there are too few to have much effect on the war in general. Might take up more engineering effort to get it into a tank turret than what the would get out of it. Granted that did not stop the Germans from devoting a lot of time and effort into prototypes that went nowhere and could be see to be going nowhere. A taper bore MK IV tank would have been an anti-tank specialist indeed as there may have been no HE round for the gun. While a practice round is listed an HE round is not in common sources (copied from each other?). Gun is a bit longer than the standard 7.5cm/L48 gun and the ammo is bit longer.
HE round for the 75/55 was supposed to look like this:
Perhaps the 5cm HE shell might've been adopted as the HE shell here?
The practice round was still good for excellent penetration figures, despite it being made from soft steel; the chance to hit was probably awesome with the speed it had and the small drop.
With the ban on carbide cores being put into effect in the summer of 1942 and the gun being worthless without the cored ammo things get very iffy. A MK IV with a standard 7.5cm/L43 gun in the Spring-summer-fall of 1942 could fire Pzgr 39, a few left over rounds of Pzgr 40, HE, smoke(?) and perhaps a few shaped charge rounds?
Yes the carbide core ban was not universal and exceptions were made. But the 75/55mm gun had to use tungsten carbide for every service (combat) round fired and each round was going to use up almost double the amount of tungsten as a 50mm round would, let alone the 30mm/37mm rounds used in the aircraft anti-tank rounds. Ground 37mms may have seen issue fade to nothing as a lot of the time the 37mm ground gun could not kill the big Russian tanks even with an AP 40 round.
What was not made for the 75/55 was the 'simple' APCBC shot - basically the 57mm zis-2 on steroids. It would've been good for taking on the T-34 beyond 1500m, and against the KV to beyond 1km; even better if the side shot opportunity arises. Not bad for a relatively compact gun that can use no tungsten and has the flat and fast trajectory.
Perhaps make a shot that has just the 1st 1/3rd of the core being from tungsten carbide, the rest from steel?
Also - try and test the APCR shot from the 5cm pak 'wrapped' in the metal container so it fits? Will use less tungsten than the normal pak 41 APCR, and will have the higher MV (that is both good and bad thing here).
WRt, the tungsten use, same thing as with the 28/20mm gun might've been applied to the 37mm getting any APRCs after 1941 - were they actually worth it on the Eastern front? A tungsten-tipped 75/55 will make for an excellent anti-tank system, even the 42mm will be useful, but to me the 37mm APCR,
unless it is the biggest AT weapon the platform (basically, an aircraft) can handle, might be an example of trying to gold-plate the wood.
So a lot depended on what was going on (in some peoples minds) in the winter of 1941 and spring of 1942. Was sticking the big taper bore gun in a tank (MK IV at the time) a good idea or not? It was Nov 1941 when they decided to put the 7.5cm/L43 in the MK IV G and shortly after that decided to put them in the last of the Fs as an emergency. First production MK IVFs with L43 guns left the factory/s in March 1942. The 75/55mm gun was a for sure tank killer, but it could not do anything else. How much time to redo the mounts for tank use? What modifications to the turret?
One source says in the Summer of 1942 (June) the Germans had approximately 170 MK IVs with long guns. How many less if some had been fitted with the 75/55 guns?
There was a lot of Pz-IVs with the short gun in 1942. Have the battle-damaged examples being the 1st recipients of the 75/55 gun?
A platform where it should've been perhaps easiest to install this gun in 1942 would've been the Stug-III.