Marrakai,

You are correct regarding the back thrust. However, there is more to consider. For example, steel has some elastic properties that are much like Silly Putty. You can give Silly Putty a slow pull and it will stretch a great deal. But if you give it a rapid pull it won't stretch, it will break. The pressure curves created by firing shotgun shells and rifle cartridges are very different. The rifle cartridges have a much steeper and higher rise in pressure. It is hard to argue that firing the rifle cartridge isn't working the steel more than firing the shotgun cartridge. You may not have exceeded the elastic limits of the steel in your guns but there can be no denying that you are getting much closer to those limits than if you were just firing shotgun shells.

We understand the pressure/area differences between the rifle and shotgun shells and the effect on back thrust. Now consider that the rear of the small diameter rifle case distributes its high pressure back over just as small a portion of the action face. That little area takes the biggest hit. The area surrounding a shotgun's firing pin holes is the weakest area of the face. This area can be reinforced by installing hard steel bushings where the holes are located, but most people making shotgun-to-rifle conversions don't go through the extra work and expense to do this.

Firing overcharged loads, firing in extremely hot or cold environments, firing through a dirty/fouled bore, firing many shots in rapid succession, and all kinds of other situations will subject a firearm to extra stresses. All guns are "overbuilt" to provide a margin of safety for these types of extreme conditions and, of course, some firearms are "overbuilt" more than others. Converting a gun built to fire 11,500psi cartridges to one that will fire 42,000psi cartridges significantly reduces any margins of safety the manufacturer built into the gun. What you get will shoot --- but so will a zip gun made from a piece of galvanized pipe with a cap screwed over the end. The whole discussion boils down to this: What level of risk are you willing to accept?

Grenadier