In X-Com terms, it's calibre 33.
Depending on your preferred measurement, a 33 caliber is either something like this
or this
More seriously, XCF does have both 5.56 and .308 ammo and most game rifles fall somewhere around that range.
So, what
kind of a hunting rifle is it, that is,
what do you hunt with it? I guess elephants are out, but is it rabbits, coyotes, deer, boars/hogs, moose/elk, what? That governs what its actual performance should look like. The description just says it's not very powerful compared to military guns, but those have quite a bit of variance, from ~5mm to ~13mm.
It would require different ammo anyway, and I'd like to keep the same clip.
Er, damage is indeed a property of ammo, but accuracy scaling isn't. We already have exactly this kind of distinction with the Mosin and the BA rifle, one being scoped and the other not.
an additional advanced optic to be added to existing sniper rifles (improving the aim, for example) (not sure this is easily doable with OXCE)
It's possible, but not easily, and hell on the modder. :'( Arsenal Additions does have a whole bunch of weapons like this, like a scoped Magnum, a Magnum with a bipod, a Magnum with both, one with neither, etc. It's Fallout 2 all over again.
But not tactical sniper rifle, it sucks if you actually want to move as well and use a snap shot.
It's supposed to suck that way. The problem is rather that it isn't commensurately better at doing aimed shots, either.