Way too low budget

Tour of Duty had Army co-operation in its production and filming. Go Tell the Spartans did not. And they were done some 10 years apart when public acceptance of Vietnam themed movies had changed dramatically. As far as the weapons for advisors goes, I have seen many photos of US Advisors (non SF) from that period carrying the same weapons as the ARVN troops they advised. Filming in California is something endemic to war movies since the industry began. It's cheaper to find a semi decent location nearby rather than go across the country or planet to find a closer match. When this movie was made, I dont think ANYONE in Hollywood was interested in adequetely financing a Vietnam War movie so soon after the war ended. This movie, while pessimistic in its' outlook, actually showed that the professionals in the Army who were hands on involved, had an idea of what they were doing and how it could be accomplished. But in the immediate aftermath of the war when this was filmed this was not an idea widely accepted by the American public. Other films made within the next few years, The Deer Hunter, Boys in Company C, and Apocalypse Now, tended to be very negative in their portrayal of the career/professional soldier/Marine charecters.

From what I understand, the budget was so tight, the filming schedule had to be cut from 40 days to 31, and 5 of those last 31 days were financed by the star Burt Lancaster ($150,000).

You can pick up on that probably under the trivia of this film, if not certainly under Lancaster's trivia.

"Also not filming in California would add to realism."

At least it wasn't as bad as John Wayne's "The Green Berets" where the troops are fighting in a forrest full of Georgia Pine trees!