One thing you might consider - it is (somewhat) possible to chamber a 300 round into a 5.56 chamber. The 300 uses the 5.56 as the mother brass. Too many documented cases of experienced people blowing up their 5.56s with a 300 round. http://notonemoregunlaw.blogspot.com...blk-irish.html Other than that, it is a VERY interesting round.
I also have a DD 6.8 SPC II, which I'm desperately in lust with. However, I use it for longer ranges - not close in SD/HD situations. So, I'll agree with several others with the suitability of the 5.56 for HD.