Because it never happened to you does not mean that it doesn't happen. It's actually not uncommon, at all, and you may be the only person in the shooting world that hasn't heard of it ever happening.

And yes, knowing what the differences are between a rifle frame and a shotgun frame, and understanding the forces involved at the time a break-action gun is fired, definitely causes me to believe that building centerfire rifles on shotgun frames is potentially very hazardous. It's interesting when someone says that X shotgun is the perfect base for a rifle conversion because of its strength or because it's got a crossbolt (that may or may not actually be functional). Upon exactly what knowledge are these claims of strength made. Strong as compared to what? "Good steel" is another one. What is "good" steel? How does one recognize "bad" steel? Any metallurgist would give his first-born for the ability to analyze steel simply by looking at it.

Rifles use small diameter firing pins primarily to provide more support for the primer cup during ignition. A shotgun's large-diameter firing pin bore allows far less support for the rifle primer. The shotguns larger firing pin bore will allow the rifle primer to extrude into the firing pin hole as the pressure in the primer pocket builds during ignition. This is commonly known as cratering is not necessarily a sign of excess pressure. In fact, there are NO visual signs that DEFINITIVELY point to excess pressure. All "classic pressure signs" can be explained by factors other than excess pressure. The ONLY definitive way to determine chamber pressure is to measure it.

To use cratered primers as an example, that condition can, and is very commonly caused by, a weak mainspring. If the mainspring is weak enough it can directly contribute to a punctured primer as the primer cup simply extrudes into the firing pin bore until it fails. Rifle primer cups range in thickness from roughly .020" to .025" and they also vary greatly in hardness from one maker to another. Rifle cartridges generate considerably more pressure in the primer pocket at the moment of ignition than do shotgun cartridges. This is partially because shotshell primers have much more internal volume than do rifle primers.

Have you ever noticed that often, a rifle primer is seated below the case head before firing and is flush (or slightly proud) after firing? When the firing pin strikes the primer, the entire cartridge is driven forward until it stops and then the pressure building in the primer pocket drives the primer rearward, into the breechface. The expanding powder gasses then drive the case rearward, seating the primer flush.

From the above, it should be very clear how a primer can fail. Another common cause of pierced primers is a damaged firing pin nose. This damage can take the form of chips, erosion or simply wear.

As far as the strength of a shotgun frame is concerned, ask yourself this. Can you calculate the shear strength of the locking lugs that is needed? How about the strength of the locking lugs of the barrels you have? How about the resultant force placed upon the bites due to the bore axis and hinge axis offset. How about the strength of the frame at the standing breech/action bar junction? That's not simply tensile strength as there is a torsional component to consider. Do you know SPECIFICALLY the materials of the parts with which you're working? How about their heat-treatment, or lack thereof?

A few dozen or even a couple hundred rounds fired without mishap is hardly proof of success. What do these creations look like after a thousand rounds? What does the breechface around the firing pin bores look like after 500 or a thousand rounds? I would think that 1000 rounds should be a reasonable minimum lifespan for a rifle of any type.

British double rifles are proofed with oiled cartridges in order to MAXIMIZE breech thrust and therefore test the strength of the frame as well as the barrels. I wouldn't want to try that with a converted shotgun, especially one of questionable pedigree, as some of these conversions are.

Much talk is made of the need of a "third bite" as if that is some sort of "magic bullet" of safety. The vast majority of shotguns with the vaunted "Greener style cross bolt" show considerable clearance between the crossbolt and the hole in the rib extension, meaning that it does nothing, at all. A whole lot of real double rifles are made with no third bite at all. A crossbolt, even if it's actually functional does not make up for material and strength that are lacking in the breechface and action bar.

A little knowledge can truly be a dangerous thing and do-it-yourself engineering is fine as long as only the do-it-yourselfer is the one taking the gamble.