Factual error: The scene where Jon Wayne's Son is shooting the deer at 600 yards, he swings his rifle about 20-25 inches in about 2 seconds - by my calculations the deer must have been travelling around 250 mph.
Factual error: The semi-auto handgun is not a real Bergmann 1911. It is a Walther P-38 that was modified to look like a 1911 Bergmann, but when Michael shows it, he puts the magazine in the grip rather than in front of the trigger, as a Bergmann would require.
Factual error: The motorcycle that Michael McCandles rides is far too modern for the era of the film. It has twin coiled rear suspension and offroad tyres on it from a more modern 60/70s era.
Answer: According to Wikipedia, the shootout at the ranch, both Devries brothers are killed by Jeff McCandles, before being shot himself. The rest die at the end of the movie.
You only see the older brother shot by Jeffrey before being shot himself. I only assumed young Billy stayed behind to help his brother.