Major General Julian S Hatcher, Ordanance Dept, US Army, stated a friend once told him the Navy dept had proved a gun did not start to recoil until the bullet had left the bbl. General Hatcher replied;
"Be that as it may, EVERY GUN, Large or Small, Army, Navy, or Civilian, STARTS MOVING BACKWARD AT THE "EXACT INSTANT" that the bullet "STARTS MOVING FORWARD"".
How many proofs does it take?? Revolver barrels point below the target prior to firing, because the bbl rise brings them on target. Dbl shotguns & rifles have their bbls converging because the bore axis is off the C/L so they tip out sideways before the bullet/shot leaves the bbl. Drawings from the L C Smith plans book show a 30" 12ga bbl's axis to cross @ 6ft from muzzle & to be 15.45" apart at 40yds. Understand this is "Not" where the shot hits, but the mechanical extension of the bore centers from a stationary gun. "The Gun Moves" when the projectile moves.`If you really feel like delving into this i would sugest obtaining a copy (And Studying it) of Hatcher's Notebook.
Someone fed you a line.