From "Flying guns" by Tony Williams:
There were various possible locations for heavy cannon, but they all boiled down to two basic types - wing or fuselage mountings. Wing mountings had several disadvantages. First, they suffered the usual problems with this location of harmonisation; that is, the guns had to be angled inwards to coincide with the sight line, and this could only be for a specified distance. At much shorter or longer distances, the projectiles would not strike where the sights were aimed.
Incidentally, it is worth mentioning that with all guns, wherever mounted, there was also a vertical harmonisation issue, in that projectiles followed a curved trajectory so at much shorter or longer distances they would strike above or below the aiming point. However, this would be much less of a problem with a high muzzle velocity. Other disadvantages of wing mounting peculiar to heavy cannon were, obviously, that the weight and drag penalties were much greater than with one fuselage-mounted gun, the wings were more flexible not just in flight but also under recoil (which affected accuracy) and the plane could be moved from side-to-side by the recoil if the guns fired at different instants, further affecting accuracy.
The guns were also mounted under the plane's centre of gravity, which meant that recoil pushed the nose down on firing.
All of this added up to less accuracy and lower aircraft performance with wing-mounted cannon. This is significant because most single-engined aircraft fitted with large cannon had no option but to fit them under the wings, as few vee-engines, and no radials, were compatible with the engine-mounting of the gun. In theory, a gun could be mounted under the belly of the plane and synchronised to fire through the propeller disk, but in practice this became more problematic as the size of the cartridge case increased because of the variations in the burning time of the propellant (the Luftwaffe considered but rejected such an installation of the MK 103 in the Fw 190 for this reason).
Of course, none of these problems bothered twin-engined aircraft, which were able to employ a rigid central mounting in or under the fuselage, directly under the sights, to the great benefit of accuracy. They also usually had a much better forwards and downwards view (important for a ground attack plane), although blocked to the side by the engines.
From Wikipedia:
Il-2 with NS-37
Referred in west as "Il-2 Type 3M". Based on two seat Il-2, armed with Nudelman-Suranov NS-37 instead of 20/23 mm cannons, this version is an approach to anti-tank airplane, prepared for the Battle of Kursk. However, combat effectiveness was quite low and production of version was limited to about 3,500. Moreover, bomb load decreased from 600 kg to 200 kg. It was replaced by conventional Il-2 attackers armed with cassettes with cumulative bomblets.
That's why your idea wouldn't work, Dave. It was tried IRL by both the Germans and the Soviets, and both found out through bitter battlefield experience that wing mounted heavy guns simply were not effective because they shot all over the place.
So how did Rudel kill hundreds of tanks? He fired single shots. And corrected his aim after every shot - not that he needed to do that often, since he had the nerve to get down to point blank range before shooting.
And apart from that...look at the critical shortage of tungsten and molybedenum in the late years of the war. No, the way to go is vanilla hard steel armour piercing bullets.
(to be continued)