There are no "factory" options... the only aftermarket stock that I know of is the Choate PG, I don't think its any shorter than the Mossberg one....
There is a person over on ShotgunWorld.com that was able to shorten a synthetic one to 12" LOP or so, he claimed it shot well, but the results were not cosmetically appealing...
If your mechanically inclined, it would be easier with a wood stock, you could get it down to 11" or so...