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