Firstly, I have a 7x65R in a break-open gun!

What I think is the origin of this warning is that, it is commonly given advice for break-open guns that, if shooting cartridges of either 7x65R or higher recoil, that only steel actions should be chosen. Dural actions, (i.e. lightweight aluminium actions), are considered to be more prone to eventually shooting loose with 7x65R and up.

This is a generalization and some Dural actions will have steel inserts to reinforce critical areas. Also, some of the new Blaser firearms, for example, have more elaborate lock-up systems. So I'd take this as advice to keep in mind rather than an unbending law of physics. But if you are looking at a classic Drilling, for instance, and the action is Dural and the cartridge 7x65R or more, I'd certainly think about it.

This info was given to me by a friend of mine, Klaus, who is currently at the German IWA show and who imports older European guns.

My steel frame 7x65R is still 100% tight, incidentally. It has an old-silver finish which suggests aluminium but a magnet shows otherwise. The 7x65R and it's rimless twin the 7x64 are very similar in capacity and performance to the .280 Remington and not too different from the .270 Winchester. So there's nothing wrong with the cartridge. In fact, I recommend it.