Off face is an improper relationship between hinge pin to standing breech and the hook to barrel face dimensions. The most common cause is too many opening and closing cycles for the hook/hinge pin. As the barrels come off face, there will be a "battering on firing" element that accelerates the wear and moves the barrels further off face and with fewer cycles.
The two most important elements are to maintain good, clean lube of the hinge pin/hook joint and to use only loads that produce pressure within the gun's design.
ps: all caps is construed by some as shouting.
pps: If interested, come on down to the Carolinas Vintager's meetings in Charlotte - lot of double talk there!!
