Piper you have a point re the frictional force in the barrel.
Overall the locking of double guns seems to be a result of empirically derived choices rather than analysis.
Over the years I have measured many guns. Some have cross pins of 8 millimeters and are solid, others have 10 millimeter cross pins and rattle. Obviously it is hard to isolate the causes of the looseness, or to prioratise them, ie does friction contribute more or less than strain from firing, etc.
The impression left after much measuring and comparing is that the fewer locking elements there are, and more generously dimensioned they are, the more robust and long lasting the lockup. Prime example is the Winchester Model 21 which as far as I can determine has a two point lockup- the cross pin and the round locking bolt. The French Manufrance Robust is another solid locker with two lock up points, so is the Beretta 626.
