Not to completely disagree with you, because you had the qualifier, "likely," but...
There have been quite a few times the Guard has fired upon American citizens. Waco, Kent St., and Watts come to mind.
5.56mm
This is true; while these are precedent, I do wonder if there would have been different outcome if they had even a vague idea that they were going specifically to "put down resistance" rather than "maintain order" as they were in those incidents. (Yes, arguably putting down a resistance is part of maintaining order; except there's a difference between "nobody passes this line, use deadly force in necessary" & "the people are rebelling; kill them.")