If I may be so bold, my own rules for gang warfare, Mean Streets: War in Gang-infested Cities, has rules for "Peaky Blinders" type action. Although written for a romanticized look at street gangs (aka "The Warriors"), there are rules for a variety of firearms.
They're $20 print, $10 PDF, so an easy price point to take a look and maybe use as inspiration for your own ideas. Because us wargamers have never bought a set of rules we thought we might no use, right?
http://www.firstcommandwargames.com/mean-streets.htmlFeel free to follow up with any questions you may have...
Mike Demana