.44 magnum is suggested by Referee's manual to have damage 2, crit 2, armor +1. This kind of makes sense when you think about in the sense of big, powerful revolvers and doesn't really unbalance things that bad because revolvers have terrible range and mag capacity.
But there is of course rifles chambered in .44 magnum, notably reproductions of wild west style lever actions. A rifle with a decent range and 10 or so rounds in the magazine would be incredibly lethal, far outperforming 5.56 and 7.62 (at least against unarmored targets), which seems slightly absurd.
Dam 2, Crit 3, Armor +1 seems to be good place to put it, with worse range than 5.56.
Of course that's where .45 ACP is at in the core rules, and reasonably where you'd put .357 magnum (a far more common caliber).
Currently I'm considering dropping .45 ACP to 2/3/+2 and putting .357 at 2/3/+1 but I have no idea where to put reasonably put .44 magnum.