I think they kind of played up the Tommy Gun more than it was actually used by either B&C or Hamer's crew (fellow ranger and sheriff deputies). I don't think B&C used it much if at all though some of the other outlaws they sometimes hung around with did. From what I have read they used cut down BARs far more often. So it was fairly good on the guns but some "loose interpretation" of the facts.
Also, I believe the gun shop scene was total fabrication. It would have been nice to have more realistic portrayal of the source of guns for both LE and the criminals. B&C and many other top criminals stole their automatic weapons from NG armories and police stations they raided. That would have been a good action sequence as well as factual. Hamer and probably most LE at the time drew advanced weapons also from the armories or from gun shops which provided them only to LE. While technically anyone could buy and own any automatic machine gun or rifle, the fact is that some of the most iconic ones were not generally available in any old gun shop, especially in small towns.
Does anyone know whether it was based on fact that Hamer's Ranger buddy was such a poor shot? I've heard that he was not a drunk or deadbeat but was actually working as a deputy somewhere and was a very fit and good Ranger and deputy. I can't imagine that someone would have made it into the Rangers if he was a poor hot since they often had to either work alone or face down large numbers of adversaries with just a few Rangers. I couldn't see what type of revolver the Ranger buddy had, any ideas?
I am reading a biography of Frank Hamer and only up to 1916right now. But most of the time whether Ranger or city marshal he had to supply his own firearms and transportation and the Ranger pay was much lower than most county or town deputies. So he had pretty basic firearms at that time anyway: Colt .45 and Win 94. On the B&C mission he must had much better logistics support from the Governor!