One phenomenon seldom discussed by us shotgun lovers is the Poisson effect that manifests in pressurised thick walled cylinders. The breech end of shotgun barrels is a thick walled cylinder, the walls being thicker than one tenth the diameter.
A thick walled cylinder contracts lengthwise and expands radially in response to pressure and then reverts to its original length. This is a fast process obviously and happens at the same time as the rearward flexing of the action. It is reasonable to assume that the two processes, chamber shortening and action flexing and the recovery, pound the chamber ends of the barrel thus creating a gap.
In some guns you can sense the pounding by feeling for the raised burr along the chamber ends.
The above is I believe the cause for coming off face and not the wear from opening and closing. But it is a personal opinion, not widely accepted.
