comparisons
Blatently declaring that the 30-30 is marginal, and anything less is inappropriate, is a bit much and a bit of trolling. But I'll bite. By doing so, such calibers .44 and .357 mag, .44-40, ( carbines ) are also insufficient, as well as the .45 cal muzzleloaders with roundball or sabot, and .50 cal saboted (.429 slug) muzzleloader loaded light, etc.
Folks are having far to much success on whitetails, hogs and small bears with any of the above combinations, as well as the x39mm round, to support such an arguement. Proper bullet placement (good shooting) and bullet construction are the first keys to success on game, and the 30-30 and the x39 are no exception.
While I do not believe that a .223 is an ideal whitetail cartridge and there are better choices, I also am of the opinion that many folks are severely overgunned when it comes to whitetails. Whether I shoot my deer with a .44mag or x39mm carbine, or .243, .270, 30 '06 or a .308, if I shoot well and use an expanding bullet, I get a deer on the ground anywhere from DRT to 50 yds or so (sometimes more) regardless. Yes, the wounding sometimes seems more severe with the "real" deer cartridges, but the reaction and results with the animals are the same. If I had a tape all the shots and reactions from whitetails that I've taken with the above mix, there is no way whatsoever one could tell what rifle/caliber I used. Some drop at the shot, some do a death sprint, all end up in the freezer.
Ethical limits for the x39? Game up to 250 lbs, maybe 300 lbs, range about 175 yds, given a rifle that will group and with a worthwhile zero.