He shouldnt be using heavy steel loads in a gun not designed for it. A double gun coming open when firing is due to the angle of the locking engagements being too severe, and not being locking tapers. If angles are too severe the mechanical forces applied to the locking engagement upon firing will overcome the angle and mainspring. A weak mainspring could also be a part of the issue, although in my opinion stiffening the mainspring is a bandaid. Consequently of angles arent enough you have nothing/little accounting for wear.
