This slight fault can come up a few years a gun has been re jointed, or is just a case of normal knuckle joint ware. Just to make certain that the gun is correctly on face there is a little used trick of the trade that was commonly used but has now fallen out of use not because it was not a good method for a quick check rather that the product used is now not so easy to obtain though if you know where to look you can still purchase a box for little cost. If I said typewriter carbon duplicating paper would you folks know what I was talking about, if you do a piece covering the guns breach with the duplicating surface facing the barrels and the gun closed it will leave marks on the barrel face that is tight against the breach face. It is not a substitute for the traditional smoke lamp but a quick way of getting a yes no status of the barrel to breach fit.If the breach fit is passable the fault can be found with the fore end iron because when rejoining the the pressure that is normally applied by the Iron to the knuckle joint is increased making the joint become tighter and is released by filing the the barrel catch slot in the Iron. Now if this slot is not filed correctly and leaving a little bearing surface the constant on and off of the guns fore end, ware here will cause the knuckle joint pressure to lessen causing lateral movement when the breach is open. This sort of movement is also caused by normal ware and tear, the cure is simple but it does not appeal to some folks because it involves a punch and a hammer. The late Jack Rowe and Larry Potterfield made a short video that can be found on u tube showing how to improve the irons tension on the guns knuckle joint. Though like others have said if it is not much of a movement leave well alone.