Back in the late 70's and early 80's I did a ton of research on the evidence (then available) on the subject. Much of the best research I found was Dale Titler (Book: "The Day The Red Baron Died"). he was doing interviews with those who were on the ground and air that were eye wittnesses on that day. Many of the Australians (MvR flew right over their guns) were getting old and passing away so there was, a kind of immediacy as to getting their stories.
The doctor that did the autopsy concluded that the .303 bullet entered his right side and continued to the spine and then exited about 2 inches of the left nipple. That's a pretty nasty thorasic would and most organs, including the lungs and heart are in that same line. Back in the late 70's the best medical opinion I could get said that death, while not instantaneous could have taken up to 2-3 minutes from the time he was fatally shot. The bullet would to his leg was considered inconcequential as it did not hit an artery.
Titler (who I interviewed on the phone numerous times) was of the opinion that Franklin and Popkin's machine gun was the dealer of the final bullet...however...he was insistant on the fact that the ground forces, in the area of Sally-le-Sec that day used .303 rounds in both machine guns as well as rifle modes...in other words, it could have been an extremely lucky shot by a soldier (most likely Australian) on the ground that day with a rifle. In other words...the man who did the most lengthy, exhaustive, detailed, research including all possible angles of fire thought that the angle of the fatal would was indeed fired from the ground and not from air-to-air combat and could only narrow the possible "smoking guns" to two or three possibles.
I met and spoke at length with the von Richthofen family gardener (Schweidnitz, Silesia--Now Swidbonitza Poland) but never spoke about Manfred's death out of courtesy.