As the hinge pin wears or the action is
overstrained, the barrels can come away from the action face,
i.e. off face.
You should not be able to place a feeler
gauge between the action face and the barrels and close the
action. You should
not be able to see light between the barrels and the action or
for that matter see any gap at all.
You can do a couple quick and simple tests
to help detect if a gun is off face without using gauges:
the top-lever hard over with the thumb and hold it there,
while supporting the barrels in the closed position. The tip
of the index finger is then placed where the barrel and
standing breech meet; then try to move the barrels moved from
side to side. This should probably then be repeated with the
forend removed, taking care not to drop the barrels!
You should feel no movement.
With the gun upside-down and horizontal
and with the forend removed, hold the gun firmly by the wrist
and/or buttstock with both hands, while the top-lever is
pushed hard-over give the gun a
gentle shake from side to side.
Again, you should feel no movement.
An off face condition can be repaired and it
is not all that expensive to fix. The barrel lug hook can
be "spray welded" and refit to the pin.
A poor solution which has been employed is
to peen the pin or hook in an effort to deform the metal, forcing
a tight fit. Another is to place a metal shim between the
hook and the pin. Neither of which are long term solutions
and should be avoided.