Guns of Navarone

Hello everyone, I am new to this forum and this is my first post:

I entered The guns of Navarone in imfdb and received data on many of the small arms in the film. However there was no mention of the stars of the movie: the rail mounted cannon and subject of the movie.
Can anyone ID these weapons?
They look similar to the 50 calibre - 16inch guns found on Iowa class battleships, but if they are German / Krupps they probably have a metric designation.

This bit of trivia has eluded me for some time, so any help would be greatly appreciated.

Sincerely,
gesshots