Naturally there is rearward pressure and no discredit is intended to anyone. What is not being analysed are the phenomena that occur simultaneously with the rearward thrust.
On firing the case expands and sticks to the walls of the chamber. Does the backthrust then come simply from the case head or the case head plus the barrels?
During this phase the chamber expands radially, it also contracts axially.
The action flexes.
And then there is a recovery phase.
The action flexes back to its rest position as do the barrels. So far I have not come across any authority mentioning the recovery of the action from flexing and whether that recovery delivers a blow to the chamber ends of the barrels, especially if the barrels are also recovering from their axial contraction at the same time. Could this be the cause of the swarf raised on the chamber ends of some barrels? Or the burnished hammered look on some chamber ends and the marks made by soft barrels on hardened breech faces?
The pressures applied by the action recovering from its flex must be considerable. Does this slam the barrels between breech face and cross pin hard enough to alter them dimensionally and put them off face?
These are valid questions that the traditional descriptions of the initial back thrust phase, and nothing else, do not answer. Considering the drop in popularity of SXSs they will probably never be answered and they are more relevant to repair than construction of new guns.
