I am happy to see that you seem to be assessing the situation correctly, or at least in such a manner that you perhaps will not get injured. W. Ellis Brown, the chap who wrote the book on shotgun to double rifle conversions, now in its second edition, tried several times to convert one 16 ga. to 7x57R, but eventually decided against it, after experiencing problems similar to what you relate. He concluded that the 16 ga. was simply too small for such use. Prior to Ellis Brown's writing of his book, I took class from him, at gunsmithing school in MN, and one of the lead instructors in the school, said, regarding converting American made shotgun actions for double rifles, that they, American double shotgun actions, are plenty robust for such use, but severely lacking in their bolting systems, so they warned against such use. In my conversions, I have never had one fail, but I use a great deal of caution in choosing the shotgun actions I use, and use only ones with well fitting Greener crossbolts and Purdey underbolts (if not well fitting, I make them so), and plenty large in size, and never an American made action. In that process, I have used mostly modern actions, of 1960s or newer vintage, but some after WWII actions, mostly German, but some Belgiam, some Spanish, and even a few French, including the big Spanish 10 ga. mag. double shotgun to full load .577 NE, without a problem, and know of at least three others who built .577s on same type actions, and two who built .600 NE on same type action, so that is plenty strong for these big bores!