Best tank-busting armament

[delcyros]

It all depends on the circumastances of impact. And of course on the projectile properties. AP-solid and/or APC shots made of low grade steel were common to soviet A/C guns. In so far they were inferior to the US rounds, but not by much.
The 45mm figure fits well to a plate net impact obliquity of 30 deg. Sadly not all such circumstances (area of hit into the plate, true impact obliquity, plate properties[elongation, brinellhardness, thickness, SS], target angle, true impact velocity) are noticed in test results (sometimes plates have been repeatedly striked, suprise, they achieve penetration!), this makes it impossible to compare the results with other ones.
I did a lot of digging into this matter and so far, I believe M79APCLC is the best penetration system out there. Thousends of plate properties have been analyzed to rule out the physical background of penetration processes for homogenious armour.

[delcyros]

The strange thing with AP-shots is that they usually are solid. So the momentum of impact and the impact obliquity defines whether or not the projectile will defeat a certain plate or not. The differences in nose shape from very pointed via blunt shaped to flat nose are significant, but not by that much (blunt shaped projectiles tend to have a better penetration ability at high obliquities for tradeoffs in low obliquities). Other than by extreme differences in plate properties and / or projectile properties, I cannot explain the problem. Even a flat nose 37mm projectile of 1.67lbs would defeat a 45mm US standart plate at 0 deg. with any striking velocity bigger than 2078 fps (627 m/s terminal impact velocity equals a flight distance of ~1500yrds, energy/unit of hole volume:37498 foot pounds/cubic inch). Assuming a plane makes 300 Km/h, the initial velocity of the projectile is 973 m/s (3241 fps), which would be sufficiant to penetrate 90 mm of US armour grade material at 0 deg. impact obliquity for a flat nosed-blunt shaped projectile (energy/unit of hole volume: 45585 foot pounds/cubic inch)! The NS-37 actually has no flat nosed projectiles, so penetration at 0 deg. should be higher. Hence, even if we assume the test plate is of extraordinary strength (which may be the case for extra hard 45mm plates), the projectile will initially defeat 81.8 mm