I don't know about SBS doubles but I have had a 9.3x74R drilling that was hard to open after using the shotgun barrels. The 'smith told me the problem was that I'd accidentally dry fired the rifle barrel and the long firing pin had got stuck. Leave a case in the rifle barrel now when clay target shooting and no more problems. This gun has had several hundred very stiff 9.3x74R loads (Uncle Nick's or hotter - Oz shooters will know what I mean - 375 Flanged Mag level) through it with no probs as well as a couple of boxes on 1 7/8oz three inch mag shot shells.

However, with the rifle barrel on the bottom, the action is probably less stressed by the back thrust - I wouldn't be surprised inf the 12ga 3" gave more back thrust.

mike