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The x39mm round works fine for whitetails and I'd suspect hogs as well, within its effective sport range of 200 yds or so. Two of the biggest whitetails I've ever killed were with a bolt rifle so chambered, both were under 50 yds. Bamaboy and I have used it on other meat deer as well. The x39mm is often compared to the ol'30-30 and though a tad behind it in horsepower, it's a valid comparison. I believe you are on the right track moving up to a modest .30 cal as a hunting round, for medium game.
No experience with the .300 Blk, but it appears more of the same, almost. A moderate weight .30 cal slug near 2000 fps. I'd expect the results would be more of the same. Its virtue seems to be that you can run it through the AR, using AR mags, with a manual of arms that AR guys already grasp. I suppose too that it can be more readily suppresed on the AR platform if that's desired.
I've got a thing about running the "other guy's" cartridge through "our" rifle, but that's me. If I wanted to hunt the x39, I'd get a bolt sporter, or maybe a Mini, and if the money was there and I wanted a "tactical" rifle,maybe a Sig556R. For the AR platform, I would not mod it out to shoot the commie round.