A pierced rifle primer vents gas under FAR greater pressure than the same situation with a shotshell. Average shotshell operating pressure is between 8,000 and 12,000 pounds per square inch, low-intensity rifle cartridges can be in the range of 30,000 to 45,000 Pounds per square inch. Why is it that rifle designers take such great pains to give an unobstructed path for the venting of gasses in just such a situation, especially considering the "rarity" (in your mind) of such and event? Your "research" mentioned nothing about the results of a rifle primer failure and is, therefore, invalid. Shotshell primer failures are completely irrelevant to the discussion since no one is going to fire shotshells in their rifle conversions. What exactly do you think will happen in the case of a failed rifle primer in a shotgun action and the venting of rifle-level pressure into the action? I've told you what, but you clearly don't want to accept it.

I do wonder about how many of these projects go wrong but I suspect that will never be answered because while it's common for people to post their mishaps when it's a factory-built rifle, I suspect that not many are as willing to share when they are victims of their own ignorance and hubris. Self-esteem and all that.

Everything that I've said is based upon actual engineering data and practices. I realize that you, being the owner of at least one homemade double "rifle", probably don't want to hear most of what I've said. I have not outsourced my cognitive abilities to Google and I don't need statistical samples to recognize poor engineering, dodgy (at best) methods or bad ideas, but since Google is clearly the authority on everything anymore, please continue as you were.