...but isn't all that far off. It was intended to offend the senses and illustrate the horrors of war. It was the ultimate antiwar film about Vietnam.
As a Vietnam Veteran, albeit as an aeromedical evacuation operations officer, I personally saw the results of this dirty war. One of my Hooch mates was an OV-10 pilot and told me about some of his missions. It was all classified back in 1969-1971, but it is unclassified today.
http://en.wikipedia.org/wiki/North_American_Rockwell_OV-10_Bronco
During that time, our government was denying any Americans being on the ground in Cambodia and Laos, but we were. My Hooch mate made routine landings in both countries. He talked about "waxing" water buffalo, strafing villages, and dropping grenades at low level for fun. There was no crazy Colonel in Cambodia, but Special Forces teams lived and fought with the Montanyards. What made Vietnam nuts was that you could never tell who the enemy were. Since it was primarily a guerrilla war, the residents of many villages were also VietCong. Our guys could go in a village one day and people would be smiling and acting friendly. They would go back a week later and it became a real hot zone. It was that kind of war that altered troops attitudes toward the people they were supposedly fighting to protect. There was a lot of crazy stuff going on. Apocalypse Now tried to illustrate that, but went about 2 steps too far.