When the gun is new it would have/should have a bearing on this surface.Its called the 'draw'.When the gun is rejointed the barrels move backwards towards the breachface and the back lump bearing comes off the draw.Some top end shotguns and double rifles have a detachable draw that can be replaced to bring this bearing back each time it is rejointed.Otherwise you just have to leave it.Most guns seem to hold up ok without it and some italian O/Us have even eliminated it.I dont know how long a large double rifle would last without a good draw bearing though and I would tend to have a look at some way of making a detachable draw in the action.It doesn't have to be large and its recess can be spark eroded in.
