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The biggest weakness of the film is that it is really two films with some of the same characters in both, principally Joker and Cowboy. There would have been some kind of investigation, and possibly it would have delayed Joker being shipped to Vietnam, but nothing is said about it in the film.
"Chicken soup - with a beep straw."

I've always been curious about the backstory on the interval between the Pyle / Hartman shooting and the next scene where Joker is in Vietnam. There must have been some sort of investigation into the shooting incident, and Joker was the sole eye-witness so he must have undergone some sort of interrogation. They might have even questioned whether or not Joker acted appropriately in the situation, as well as Gunnery Sergeant Hartman. There is no mention of this later in the film, but it must have had a lasting effect on Joker.

Because not everybody who witnesses violence gets PTSD

Firstly, we don't know how long the time gap was between the murder/suicide and when we next see him in Vitenam. Secondly, it's probably fair to expect that psychological aftereffects of such a sudden event are more easily swept aside than the repercussions of acute fear/stress sustained over long perdiods of time.
"facts are stupid things" Ronald Reagan

Cuz not all marines are wimps n csnt handle violence . There are guys fighting in Africa n other 3rd world beep holes for decades n don't cry about ptsdit's only the Americans who bitch n cry

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PTSD can take years to manifest itself, and a lot of the routines of armed forces life are designed to stop people having the time to brood. A British veteran of WW1 said officers and sergeants seemed to do their best to keep soldiers busy to stop them thinking about their last battle, or their next. That is why many psychological problems only manifest themselves later in civilian life.
"Chicken soup - with a beep straw."

There's no such thing as PTSD.
When you come home from a foreign country after killing innocent people the name of what you feel is
GUILT
.
Please correct my grammar, make my day.

That's a small minority. And a lot of them are whiney crybabies who won't accept responsibility for taking care of themselves.
Fowler's knots? Did you say fowler's knots?

Then why do so many soldiers come home and commit suicide from PTSD?

Most military veterans deal with their military experiences very well. The training is designed to prepare them for it.
Fowler's knots? Did you say fowler's knots?