Thanks, I am aware of what a recoiless gun is. My recollection was that it was vented over the side.

I did look it up and indeed the weapon was not recoiless, but as to stopping the plane:

"A B‑25 lost about 5 miles per hour momentarily when the gun is fired and if you opened the side window and put your arm on the ledge, the flash would singe the hair on your arm. I tried it several times to show other pilots the effect of the flash.

from:http://www.armyairforces.com/forum/m_70063/mpage_2/key_/tm.htm

My dad was a crew chief on a B-17 stationed at McDill Field in Tampa during the war. He said guys would fall asleep in the shade of the wings. That was until a gator wake a guy up one day.

He also tells the story how several B-17's ended up in Tampa Bay as the approach to the main runway is over the bay. Said the boats crews were kept pretty busy fishing guys out.

I did not mean it actually stopped the aircraft if flight. (law of equal but opposite reaction, you know) But my phrase was meant as a word impression as to the feeling when fired. I should have been clearer in my verbal description, thanks.