That's one of those urban legends that have been said so many times about so many planes that somehow it's become one of those "everyone knows" things. The most common is the A-10, but it's far from the only one.

Beaufighter Mk IF:
Cruise speed, 272mph. Weight, empty, 14000 lbs, max operational 21100, average 17550.

Armament, 6x.0303 machineguns, 4x20mm cannon.

Force = Mass x Acceleration.

Not even if you cut loose all the ammo out of all the guns at the same time could you make a notable dent in the amount of energy moving forward.

...!
Stoke Mandeville, Esq & The Victorian Ballsmiths
"Nobody Want Verdigris-Covered Balls!"