Here's my take on it, Tom.

There is so much friction in all the wads and shot column that there is little or no movement form recoil. This is partially due to the fluid-type of recoil absorbsion of a charge of shot, where each individual pellet is very light in weight & thus not very susceptable to movement from the jarring of adjacent tube.

Even if a shot charge loosened a bit, the wads on the powder and most of the shot would still be in contact & would not make a dangerous looseness or 'space' like that which occurs with a bullet sliding forward off the powder charge.

The tight fitting patch on a round ball prevents it's movement the same way - due to friction.

Bullets did not become popular for hunting in DR's until the ctg. era, which was actually happening in Africa and India by the mid 1860's for rifles and even earlier with smoothbores - ie: Lefaucheau SideXSide pin-fires. The reasons for this are quite likely at least 2 fold.

Fixed ammunition not only allowed the use of hardened bullets which were needed to ensure deep penetration but also prevented the second barrel's bullet from moving foreward due to the recoil of the first tube fired. Fixed ammunition thus remedied two problems, that of the poor penetration of pure lead bullets on heavy game, and that of bullet moving foreard from recoil, thus presenting an obstruction to the very fast sporting powders of the day.

Making the 'bullets' of sufficient size to prevent movement had it's own problems. Even in the late 1850's, there was considerable trouble with 'jammed' or stuck bullets if the bullets were made fit the bores so close as to prevent movement from recoil.

I've personally shot quite a lot with bullets of a size as to engrave in the rifling upoin loading and with mechanically fitted bullets. I do not think they would move under recoil or normal loads, but would not think they'd remain hard against the powder of heavy hunting loads in large bores. If of a size to do serious work, that size itself would present much increased friction and resistance to loading. Self defeating system, comes to mind.
Sorry I strayed away from shot loads.