Lots of gangsters in the Prohibition era had 1911's because they were the standard sidearm for US military in WWI and were sold as war surplus. Thompson submachineguns ("Chicago Typewriter"), favorite weapons of Chicago mobs, were not illegal in the Twenties because there was no Federal law against private citizens owning machineguns until 1933:
https://www.google.com/search?q=Thompson+submachine+gun+ad+showing+cowboy&rlz=1C1GCEA_enUS900US900&tbm=isch&source=iu&ictx=1&fir=5doir8Sc2qPYSM%252CoprgVqPdP-rf-M%252C_&vet=1&usg=AI4_-kT1IFP6NQ71aaLto1kRYBoGYBkQRQ&sa=X&ved=2ahUKEwjw_tmWgtLyAhUYHM0KHUFnAAoQ9QF6BAgPEAE#imgrc=5doir8Sc2qPYSM
Here's a couple articles about the Chicago mob and the :Tommy Gun"
Concerning Louie "Two Gun" Alterie:
The Thompson submachine gun
On a 1924 hunting trip, Alterie demonstrated to his boss, O’Banion, the new Thompson submachine gun. Local ranchers had been using the automatic firearm to protect their ranches from coyotes. O’Banion was impressed enough that he stopped in Denver to purchase three of these weapons at a “downtown Denver hardware store.” (My best guess is that these were purchased at Tritch Hardware, the most prominent downtown hardware store at the time.) O’Banion ended up purchasing 250(!) guns in total during this visit. Most of these weapons were probably the guns kept by Alterie at his Jarre Canyon properties and later at Sweetwater Ranch. But at least one of these “Tommy Guns” went back to Chicago with O’Banion and Alterie. This suggests that these gun purchases in Denver introduced the Thompson submachine gun into Chicago gangland warfare!
Also, concerning Peter von Frantzius
:
Peter von Frantzius (sometimes Frantizius) was a Chicago businessman and arms dealer to the Chicago underworld during Prohibition, later dubbed by the press as "The Armorer of Gangland".
He was the son of German immigrants. His father Fritz von Frantzius was a stock market dealer and died 1917.
Frantzius started with mail order sales of guns from his parents home and opened a shop later on. 1924 he had a very successful business selling all manner of sports goods including guns.
An almost exclusive supplier of the Chicago Outfit (although often selling to rival gangs such as the North Side Mob), he was one of the first to supply "Tommy" submachine guns and other specialized weaponry connected to countless gangland slayings during the bootleg wars of the 1920s, including the murder of Brooklyn mobster Frankie Yale in 1928, the St. Valentine's Day Massacre in 1929 and the 1930 gangland murder of Chicago journalist Jake Lingle.
On one occasion, when asked by authorities to explain the sale of six machine guns to known organized crime figures, Frantzius answered before a coroner's jury that he had assumed the weapons were for the use of the Mexican government to use against revolutionaries.