There was a great article in Air Force magazine back in the 70's I think, about the B-25G with 75MM in the Med.
The Navigator was detailed to load the cannon. The firing rate was rather slow and they could get but a few rounds off on each firing pass, which usually did not hit anything. The only way to hit anything would be in a vertical dive and given that the airplane lacked dive brakes, that was not feasible.
After one unsatisfactory series of attacks the disgruntled pilot told the Nav he wanted a course home. The Nav, stripped to the waist, exhausted from loading 75MM shells as fast as he could, told the pilot, "Fly West."
The pilot informed the Nav that "Fly West" was not a proper response and that he wanted an actual course. The Nav went to his table, consulted the maps, and replied, "Sir, don't fly West. Fly 270 degrees."
On another occasion a pilot decided to go up and try out the cannon in level flight. Not understanding the timed fuse on the shells, he would twist the nose a couple of times to make sure it was armed, load, and fire. The shell would explode right after it left the cannon, 20 or 30 feet in front of the airplane . Finally giving up and heading back to base, the pilot reported he was unable to properly evaluate the cannon because the enemy antiaircraft fire in the area was absolutely terrible.