As you say, it's very subjective. From my experience it's not really the calibre that is the deciding factor. I reckon it's rifle weight, stock design and factors such as muzzle brakes (not that I like 'em!), mercury/tungsten bead recoil arrestors and to a large extent physical build of the shooter. I've found that tall thin shooters seem to generally have a lot more trouble with recoil than shooters with my sort of build which is short and stocky/broad.

I guess one test is whether the shooter can take repeated freehand shots with their rifle - but really the only acid test is whether they develop a flinch or not. I always use my .500 Jeffery wherever I hunt and don't have any problems with it at all - but had a client last year (who incidentally was 6' 6" and skinny) and he was very recoil shy indeed.........