The first survey like that I am aware of was given to Marines, at Twentynine Palms base (a place I was stationed in years ago). IIRC about 30% or so said they would fire on Americans. There may have been other surveys since then.
Of course it is in government's interest to get that percentage as close to 100% as they can. I think they'd have some problems though. Even in Russia and China, the government could not completely depend on all troops to repress the people - never mind shooting at them. They had to shop around for the "right" units to do the thing at Tianmen Square.
