No disrespect meant to Ellis Brown, his book has some good information on building DR's, but it is not surprising that he ran into trouble with his 16 gauge to 7x57 conversion.
He describes such a conversion in his first book, and shows his calculations regarding pressures against the standing breech.
The calculations he makes are BADLY wrong, and I do mean BADLY! He grossly overestimates the strength of the 16 gauge action he is using.

I don't know if he made any attempt to correct the calculations in the 2nd edition of the book.