Just to be clear, I did not get into the old argument over .308 Winchester vs. 7.62 NATO or even into whether the pre-98 Mauser actions are strong enough for either or both.
My point was solely about the way those rifles were converted and my lack of trust of the reworked chamber, made necessary by the fact that 7x57 is longer than 7.62x51 so one does not just "ream out" the barrel to the latter caliber.
In fact, it is hard to say why or if there is any danger. The chamber insert should stay in place, but if it doesn't, I think it would act like the old "floating chamber" and come back at high pressure. I would prefer not to take a chance.