First, we have the 45 mm gun. Naturally, it can't do anything to the Ferdinand's thick side. AP shells shatter when hitting the armour, cores from APCR shells sink into the armour without penetrating.
Next, the 6-pounder from the Churchill. Penetration is achieved at ranges of 300 meters, but firing from 500 meters yields little results. Shells either get stuck in the armour or shatter.
Now, the Sherman's turn. M-72 shells from the 75 mm gun cannot penetrate the side or rear armour. The M-61 shell fares better, penetrating the side at 500 meters.
Next, the Soviet 85 mm gun. These shells are easily able to pierce the side of a Ferdinand, causing spalling on the inside.
The proving grounds also had a Panther, and decided to see if it could take on a Ferdinand. Turns out, it could. The vertical front armour can be penetrated by the Panther's gun.
The front of the casemate, however, proved too tough of a challenge. The shells made 170-180 mm dents and could not penetrate all the way,.
75 mm APCR can penetrate even this tough armour.
Penetration through the side. Note the jagged outline of the hole, caused by spalling.
Spalling damage from the inside. The large hole is from an AP penetration, the small hole is from APCR penetration.
The Ferdinand had a chance to take revenge on the Panther. The 88 mm shell ricocheted, but led to a breach and spalling in a 300 mm by 250 mm area.
The results of these trials were used to compose this guide on fighting the Ferdinand.