before we get all moist over the conclusion (though i applaud his testing) it is based on a sample size of ONE chamber labeled 223 and TWO chambers labeled 556.
in other words, statistically,what he's done is completely meaningless.
While I wouldn't really be afraid of shooting 556 in a 223, I'm NOT going to recommend it to others because it is possible to have a problem. In perfect conditions, you're probably going to be fine, but if it's raining, or 110* or you got a little dirt in the chamber, or you get some tolerance stacking that doesn't go your way, you could have a kaboom.
Also, at just about every carbine course I go to, there's some poor dude with a 556 chamber that is popping primers with factory ammo. Inevitably, someone runs one of ned christiansen's hand-556nato reamer tools and out comes a bunch of metal shavings, which means the "556nato" chamber wasn't really 556nato, but as this article points out, somewhere in between. and it's always tighter, not larger, meaning higher pressure because (my opinion) chamber reamers are wear items and can only be used so many times. if you measured the first rifle and the last rifle made by a given reamer, i'll bet you see a big difference. and if you measured them with a strain gage, i'll bet you'll see a big difference as well. my point is this also happens on 223rem chambers, and if you get a tight 223rem along with higher pressure 556nato ammo... sure, they run "proof" loads through those guns at MUCH higher pressures, but you wouldn't want to run thousands of rounds... it's asking for trouble
btw, i also have a strain gage that i tested a bunch of bolt guns with years ago. they're fun tools and the cheap ones are not that expensive. I think you'll find the graphs in the article and the ones on this page looks quite similar.
http://www.shootingsoftware.com/pressure.htm
I'd also recommend giving the dude who sells these a call. he is a wealth of info and fun to talk to.