The final shootout between Ma Barker and Melvin Purvis is captioned to have occurred in Lake "Wier" when in fact the location is near Lake Weir.
It was Freddie, the youngest son and Ma's favorite that met Karpis (Stallone) in jail and summoned him, not Doc. Freddie was in jail for killing a police officer during a car theft when he met Karpis, a gangster and career criminal. The Gang became the Barker/Karpis Gang after his arrival and the entry to the big leagues.
The father actually left Ma and the 4 boys in real life because "Ma wore the pants" and he felt emasculated so he left the family, not Ma and the boys driving off into crime and leaving him.
When they have the shootout in the Hotel and Ma gets shot, the boys run out to street level and you can see the steel door and handle aren't period correct when the door flies open, and when they show the door completely open in another shot you can see the plastic window insert from the 80's a hydraulic door pull and gold tape over the handle and trim on the inside of the door.