seems to me that you could significcently shorten the form factor of a shotgun by both shortening the barrell and casting as a bullpup.
If the chamber was in the stock to an exent, you could get a few less inches out of the overall length.
A bullpup has the chamber in the reiver, not the stock. You'd still be limited to the 26" overall length. Still it's been done for semi-auto shotguns, not much point with any break action, since they already are awfully short when the barrell is cut to 18". In fact most folks who run afoul of the 26" rule, with a "long enough" barrel, do so with break action single or double barrel, since on a pump or semi-auto the reciever adds much of the required extra 8" all by itself.
Although I have seen pumps designed for the "shorty" 12 gauge shells that are much shorter in the action than a standard model designed for 2 3/4' shells.