What idiot was in charge of the gun props?

They tried to explain that guy having an AK, but it's stupid nonetheless. You don't carry the enemy's firearm in a war; that's a good way to get yourself killed. Different firearms make distinctive sounds. An American firing an AK in Vietnam sounds like enemy fire, and is risking a hail of friendly fire in his general direction from people who can't see him but can hear his gun.

An even bigger WTF was Samuel L. Jackson's Colt M1905 pistol (or 1902 or 1907; they all look pretty much the same). What kind of out-of-left-field decision was that? He should have had an M1911 or M1911A1 obviously, because that was the standard-issue sidearm at the time and had been since before WWI. Those Colt 1911 predecessors were never issued by the U.S. military, were commercial failures (thus, uncommon), and unreliable/troublesome. The idea that a U.S. officer would be carrying one (I think he may have had a pair of them, no less) in Vietnam is absurd.

John C. Reilly's character had a commercial 1911 (as opposed to a USGI M1911 or M1911A1 like he should have had), manufactured in the 1980s or later. I guess he really was a time traveler.