I think it is a matter of choice. The boxlock may be a bit of a stronger design because not as much wood needs to be removed from the wrist area of the stock. On the other hand, because of the design, a sidelock gun is often slimmer in the wrist and action creating better handling characteristics.. It is also generally easier to change a mainspring on a sidelock should one break in the field. Most high end guns are sidelocks.
The Sidelock is the stronger and less prone to breaking stocks because most of the working gear are on the locks at the sides where as with boxlocks more wood in the stocks must be removed to accommodate the working parts of the gun.
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