This really interests me. S&W makes a .300 AAC Blackout (Also rated for Whisper) upper for about $700. A nice thing about uppers is that you can buy them direct, no FFL. Some, like Noveske, AAC, Advanced Armament and so on, make very high priced uppers while Olympic and CMMG make lower priced stuff. I think I'd take my chances somewhere in the middle with S&W. The .300 is only different in the chamber and barrel diameter. Same lower, bolt and magazines are used. This gives an accurate (and bigger) round out to a good 600 yards while the .223 is good out to 400 yards. I guess I would want a heavier buffer, too, but no big deal. I was going to get one anyway for that varmint-barreled upper from Stag Arms.

For hand loading (which I will be doing), you just convert .223/5.56 brass so it would be cheap, too! I just watched another Youtube video where a guy shot a .223, picked up the brass (HOT!), sawed the top off, trimmed and sized it, etc., loaded the bullet and fired the loaded .300 from a different AR in 1 minute, 5 seconds. I've seen other videos with guys sitting there using the case trimmer to shorten the case but this guy uses a little tiny Northern Tool chop saw before trimming to shorten the time a LOT.

Take a look, internet connection permitting, it's only 2 minutes. And yes, the guy's dad sounds like he was in Swamp People and I expected to hear zydeco music in the background but no mocking them, it's an interesting demonstration.

http://www.youtube.com/watch?v=4TUdAeyipiI