I had a good friend who flew B-25s in WWII. After the war he joined the reserves and his squadron had one of those B-25s that mounted a 75mm cannon in the bow. The breech was between the two pilots.
One day his skipper and he decided to fly the thing and shoot that gun. I don't know where they found a round. So off they went from Hamilton AF Base (north of SFO). They flew over the ocean. When they thought it was safe they loaded up the gun and fulled the trigger. He said that the airplane stopped, and the bullet just plopped out and down. It did not soar off over the horizon. He said it almost immediately fell from sight.
I doubt any of those cannon armed B-25s are around today.