The first 2/3 or so of the book is slightly fictionalized history, as seen through the eyes of a few fictional people (the Bowman's and Max). Somewhat similar to how Forest Gump was written: real events with a fictional character interacting with them, except that FG was much more involved in those historical moments than the characters in UC. The idea is to give readers, most of whom have little historical background on the origins of US gun laws, a history of them that they can relate to. By describing the direct and indirect effects of those laws on the Bowman family, you really get a sense of how important those changes really were (and are).
While not everything happened 100% as portrayed in the book, it is about 90% accurate, with the other 10% being speculation because no one really knows the exact details. Some parts, such as the '86 FBI Miami shootout, are very accurate, because there were a number of survivors and forensics to record the story. The older events have less data available, and had to be filled in and were dramatized.
The last 1/3 of the book is total fiction, of course.
-Troy