Slightly different theme here is that a B25 [and similar guns[ with little or no bite will shoot bottom barrel OK but has a tendency to come open when top barrel is fired . All to do with the height of the back pressure above the point of bolting , I'll let others argue the technicalities .As a result top fastening guns will require less force/pressure on the bolting to keep them closed than bottom bolting guns .To further add to the discussion you would also need to factor in the length of the bar , depth of lump and height of the fence .
What ever this is there is still a need for a gape between action flats and barrel be it on a side by side or an OU. Webley reckoned the ideal was .002/003" at the front and .004/6" at the back. The gape was left slightly bigger on jointing to allow "blacking down" after hardening when the gun was freed.
