John-
At this late date, it seems impossible to credit or discredit Ed Hoffman. I prefer witnesses interviewed early after Nov. 22, before "sides were chosen."
I think the Grassy Knoll smoke-and-shot show was a diversion. The medical evidence appears to be JFK was struck from behind. So even if Hoffman's recollections are correct, the Grassy Knoll gunman either missed (from very short range), never got into position to get off a good shot, or just fired intentionally smoky shots, possibly from a handgun.
Interestingly, Jack Ruby carried a snub-nose, the type of weapon best for making a lot of noise and releasing a lot of smoke.