Barrels are pressure vessels and are stressed by pressure, pure and simple. The actioning between barrels and action is stressed by cartridge back-thrust, a force, not by pressure. Said force is developed from pressure (chamber pressure) times area of the cartridge base. I'm right sure Browning considered this in the design, as 2-p pointed out. This setup has been around for quite some years and I've never heard a report of problems with the gun's strength. One point that must be addressed is that firing pins are appropriate for both the rifle and shotgun cartridges employed.
DDA
