First, let me say, that in the last three years since my first posting on the boards regarding my grandfather, Graham Barnett, I have learned a lot more regarding the circumstances of the shooting in Rankin in Dec 1931.
I now know, as you said, that Graham Barnett was indeed looking to get Sheriff Fowler, they were friends at one time for several years, but had a falling out. Fowler was exonerated in court, and it really was "kill or be killed", in fact, as your grandfather said in court. (as per the court record). Why the falling out, is not clear. Sheriff Fowler and Graham had met the day before the shooting in private to discuss business. It is reported in three different manuscipts, and newpaper articles I now have, that a year before, Barnett had sold or given Fowler a sub-machine gun which had not been fully paid for or for which there was disagreement on the business arrangement they had made. Also, it is noted that there may have been two machine guns that Fowler had, which were similar makes, one owned by Upton Sheriff's department, the other one Barnett had given or sold Fowler. In all three manuscripts it is reported that the machine gun used was the one given by Graham.
The three manuscripts I have are;
1. Unpublished biography of Graham from Sept. 1958 entitled,
"Graham Barnett: 20th Century Cowboy Gunfighter Holdover From Another Era," by Russel M. Drake, who was a resident of San Angelo at the time.
2. Unpublished biography of Graham from around 1985 entitled,
"Graham Barnett: A Big Bend Shootist,"
I thought this was authored by the late Professor Joe S. Graham, Jr. of Texas A&M, but I am now not sure. I will resolve this hopefully soon.
3. Journal article from the Journal of Western Folklore, Vol. 24, No.2, April 1965, entitled,
"The Ballad of Graham Barnett, Badman of the Big Bend in Texas," by the late Professor John Q. Anderson, Texas A&M.
I am continuing to research this and would be happy to share what I have if you are interested, and of course, I am interested in any materials you may have.