My point is that a double barrel action is not inherently weaker than a bolt action. Sure a lot of the old rifles were not built to take very high pressures but I can show you plenty of old bolt actions that will turn into hand grenades before you reach 50000 psi. Given proper design and good metal either type can be built strong enough to leave either the barrel or the brass as the weak link.

The whole notion of double barrel actions being weaker began around the time of the switch over from black powder to smokeless ran low pressures in an attempt to solve the extraction problem. It had nothing to do with the strength of the action.

Another point is that if we are designing a cartridge for a double barrel, there is no need to run high pressures. The requirement for high pressures comes about when the cartridge length is constrained by the length of the bolt action. Compare a 450 3-1/4 Nitro cartridge to a 458 Winchester. The 450 drives a given bullet weight only a tad faster than the 458 but the case is much longer allowing it to do so with lower pressures. The 458 is constrained in length by having to go through a standard length bolt action and is pushing the limits to get close to what the 450 does without breaking a sweat.

The principal stress on the action is the back thrust not the pressure. Consider the 600 Nitro. It probably cranks up more back thrust than a 460 Weatherby which is the king of the blasteroonies. (That latter term came about when my wife accompanied me to the range. Some guy was shooting a 460 Weatherby and she asked what made such a gawdawful noise and I told her it was a 460 Weatherby, the king of the blasteroonies.)

Where the double barrel action is weak is in the matter of mechanical advantage on extraction. The bolt action has a clear edge on this point.