My suggestion would be a pistol variant of either an AR or an AK chambered in a rifle caliber.
I would choose a pistol variant because the laws concerning carrying a loaded rifle are kind of weird around here. Either one (AR of AK) would be reliable and have a relatively high capacity compared to a handgun.
Even a pistol variant of one of those rifle platforms would be as or more accurate than most handguns at.moderate distances and much more effective through a car door of glass.