Well, I can't speak for the barrier penetration nor the 6.8spc ballistics. I've never owned one, nor aspired to. Its seemingly a dead cartridge, not widely available, and not something I can easily get components for. 6.5 Grendel covers that niche quite well apparently, so I wont get into a debate over ballistics at distance or barrier penetration.
What I will attest to is the hunting prowess a 223/556 has. I live in North East Missouri. The deer here are large, well fed, and have a knack for needing good shot placement with ANY bullet. Now, I've been using in one iteration or another, a 223 since 2003 for taking deer and coyotes here. If I recall, and I may have to access the DNR site for an accurate count of my tags filled, I have taken about 30 deer using a 55 gr Barnes VorTX at ranges up to and including 180 yards. I've recovered all deer shot this way, found adequate tissue destruction, and easily followed blood trails due to pass through shots. I have yet (knock on wood) to fail in finding a deer I've put a 223 into. All this, and our deer are much larger in size, weight, and hair thickness than those south of the Ozark Mountains. I cannot in good conscience say that 6.8 is "better" than 223/556. Its just another option among many, especially when its all about shot placement, adequate damage, and knowing ones limits. Had we been discussing the adequacy of 6.8 vs 556 in the battlefield theater I'd guess the 6.8 would shine for that purpose, as wounding is the name of the game, so says the Geneva Convention. But we aren't, we are talking hunting. So, while I assume you have tales of deer killed at 500 yards or better, with a DRT thrown in for good measure, I can't buy it at Walmart, the components for a rifle (barrel, bolt namely) and those needed to reload for are as hens teeth. Here, id be better off going 6.5 Grendel, which blows 6.8 away, or plop on a 300 blk upper if I felt I needed heavier bullets and marginally more energy. Heck, folks have been putting game down with 300blk with aplomb lately. And I can buy it locally, and it emulates the 762 but is more efficient, which has also taken a slough of deer over many decades in the SKS. I dont see 6.8 killing anything but my wallet. I need results, and a cartridge that puts meat in the freezer.
And 223, where legal, does that just fine.
