I was determined to stay away from this monumental exercise in mole-hill mountaineering, but there is one thing that nobody seems prepared to mention. A double rifle built on an inadequate shotgun action may well go off the face when proof-loaded, due to the stretching of the action or deforming of the lumps, but is also likely to suffer catastrophic failure due to cracking the action at the point where the action-flats meet the standing breech. Such a failure certainly destroys the gun, and is very likely to result in injury to the shooter if it happens in the shooting field. Many BP shotguns in Britain have cracked the action when resubmitted for nitro proof.

A simple inspection with a caliper and some common sense will tell you whether a gun is at risk of failure due to small lumps or thin chambers, but only proof-firing will prove the action to be strong enough. This is a back-thrust issue, pure and simple! Nothing to do with torque!

Have a look in Austyn's 'British Sporting Guns..' for photos of cracked SxS actions. I believe Boothroyd has published a few too. Sobering stuff.