You are right, of course. I doubt 5% would be willing to shoot civilians (even armed civilians) unless they were under attack or there was looting and destruction of property. And, those 5% would quickly fall in line with the other 95%. I just can't see our guys in uniform putting down a revolution of the people should it come.
Getting troops to fire on the own countrymen is, as Magna indicated, not that hard to do. The main thing is to delegitimize those who are to be shot, first. You label them "commies", "hippies", "scumbags", etc., and make the labels stick. This makes them "other", and then it's "ok" to shoot them.
I agree that dehumanizing the "enemy" is the most effective way to get someone to willingly murder another human being. This has probably been true in every single conflict throughout the history of man. What I disagree with is the "ease" you seem to ascribe to getting American serviceman and woman to murder their fellow citizens, people who look like them and sound like them. It is much easier to dehumanize a people that are demonstrably different (i.e.: physical characteristics, language, religion) but this would not be true if the army was ever turned on American citizens. I cannot speak for all, a majority, or even a large portion of those who are active duty....but the soldiers, sailors, and Marines (don't know any active duty airmen) I know have universally expressed disgust and contempt for things like the NDAA and what it portends. Without any prompting from me they have stated emphatically they will not follow any unlawful orders, and they consider orders to shoot unarmed Americans unlawful, as would I. http://www.facebook.com/pages/Oath-Keepers/108535649170589
As Max indicates with his reference to the Milgram experiment (many others have followed, that's what grad students do) it is part of the human psyche to be susceptible to this. You think our troops are "different"? Well, it's likely many other dissenting groups around the world, today, are quite surprised by the behavior of their "own" troops.
I think some of our troops would do it and some would not. Pretty sure I've been clear on this but let me state it again just in case, some troops would obey unlawful orders to murder their fellow citizens and some would not. I am unable to offer percentages. Additionally, a controlled experiment is not the same as real life. Never has been and never will be. Also, we are living in a new age where the free flow of information remains largely unfettered by the powers that be. Government propaganda no longer has the absolute power it once had. As a result of this, there is no model you can point to that factors all the relevant variables.
I can imagine an argument for the likelihood of US troops firing on US citizens being higher here, because our citizens are so well armed. It's starts with a scuffle, maybe some fists are thrown. Then someone, temper lost, throws a brick. Now, the citizens are "armed", dangerous. To be safe, they have to be herded and quarantined. That's gonna make some of the more level-headed mad, and now they're gonna resist. And so on. Anyway, back to labeling "others", you have to pull on just the right heartstrings of your troops, a version of "know your audience". Have a look back on this very forum at the reactions to the OWS movement. Delegitimization running rampant. Imagine large numbers of citizens, or more importantly, influential citizens, demanding that troops "control" these other citizens, the socialists, the parasites, the tit-suckers, the crybabies, the lazy, the unwashed, the hypocritical!
I can imagine quite a bit myself but that doesn't automatically make what I imagine realistic. As to the de-legitimization of the OWS movement, I agree that has occurred. In fact, I have personally contributed to it, albeit on a relatively small scale. However, the events surrounding Occupy this or that have been largely peaceful and to my knowledge no one has been shot to death. There were some serious injuries but what you are talking about is opening fire on a crowd of unarmed civilians. That is a far cry from anything that has happened. As long as mass demonstrations remain peaceful, I do not see the government firing lethal munitions into the crowd. Finally, what you don't seem to be factoring in is that our troops have access to information outside of what the government feeds them. The internet is a tremendously powerful tool and until the government effectively shuts that down (and that may happen soon enough with SOPA and PIPA), a percentage of the troops will be able to tell the wood from the trees.