Most folks who have not worked with a Long arm in vehicles don't really understand. Its why the M4 currently is below optimum barrel length for best ballistics. Same for shorter barrel shotguns, but it doesn't matter so much since a shotgun is trash past 100 yards anyway (and iffy at 100).
Given the choice between a short barreled shotgun or a machine pistol or carbine; I will take the shotgun any day. Total length should not be over 28 to 30 inches (go sideways through a standard inside door) and have a magazine that goes to the end of the barrel. You can do this with an 18 inch barrel but you have to short the stock. A 14 inch barrel lets you full stock the gun (a must with a 10 or 12 gauge) and still keep the overall length down.
MY 2 cents.
Well said sir.
Most law enforcement agencies now have at least a few short bbl rifles and shorty shotguns