In my realm of understanding, Hal, there is no shot stringing inside the barrel. The shot stays inside the shot cup, or somewhat "tight" against the under shot wad, until some time after exiting the muzzle. Gases are still exerting pressure behind the payload, so it will remain contained in the shot cup until exiting the muzzle where the atmosphere exerts drag on it all, and the shot cup or wad begins to lag behind due to less kinetic energy and more drag. My understanding of it, from much reading of writings of those who have actually measured it, is that all shot stringing occurs after leaving the barrel, and drag from the ambient air is one of the main culprits, and is contributed to by pellet damage, due to setback and mis-shaping, when passing through the forcing cone and choke.
