"7x65R is a killer for the break open guns"

I do not think so.
7x65R was a cartridge development for matching the benefits of the 7x64 in a cartridge suited for all kinds of combination guns and single shot (break-top) rifles.

For this purpose a strong rim was added and the pressure level was reduced in the 7x65R.

Your post was the first time I heard a comment about the 7x65R being hard on rifles.

best regards
Rolf