As a lifelong hunter and rifleman, I state emphatically that NO ONE can fire 3 rounds from a BOLT action rifle in 4 seconds and hit the side of a barn.
Furthermore, I have believed for years that Johnson was behind it. Him and his big oil buddies.
I've seen the film many times, and seen his head jerked backwards. Being shot from behind does not cause that. You can see his brains splattering out behind him.
What to look for in a case of murder,
Motive
opportunity
and who benefits most? two of the three fall on Johnson.