Considering that the ejecta consists of primer propellant, powder, wad, shot, and trapped air, you can "asume" that there is movement of ejecta upon primer ignition. The primer gas will occupy the powder space almost instantly; a small forward movement of part of the ejecta. The primer gas will have enough pressure to start compression of the wad coussion section; another forward movement of the ejecta. The combined primer and powder gas will quickly have sufficient force (pressure acting over an area) to overcome static friction of the wad which will allow the wad to act on the shot and trapped air. As you can see, movement will start virtually immediately. Significant movement of shot takes only a little bit longer. As the load approaches peak pressure it has something like 4,000 pounds of force acting on less than 1 1/2 oz (like 1/10 pound). And that, good shooters, means some serious acceleration.
Drew, I'm not sure this answers your question. If not, tell me and I'll try some other tack.
DDA
