This is a really tough issue because we are looking for something that conveys a person who doesn't just own guns ... doesn't just like guns ...
Trying to think of the kind of context where MD uses the term -- I happened to reread his piece on 7mms from a couple years ago and noted a discussion of "several" people who'd had Ruger No. 1s or Dakota Model 10s chambered for 7x65R.
Since either of those rifles is easily available in .280 Rem., which will do exactly the same thing without the need to make cases out of whatever you'd make 7x65R cases out of, I don't think "owner" or "enthusiast" quite covers it.
