You could also install one of Mossberg's "bantam" buttstocks. Length of pull = 13". It also has a tighter curve at the wrist to put your hand closer to the trigger. It's black-painted wood, though, so it won't do if you really want synthetic.
BTW, before I went the bantam route, I shortened my synthetic stock as George describes. The bosses for the screws are long enough to accomodate a 1" cutoff. I drilled the holes 1" deeper before making the cut, using the existing holes as a guide. I used a hacksaw and a palm sander, and it worked pretty well.
One problem you might have, depending on how much you cut off - the butt gets smaller, so you will also have to cut down the recoil pad, and with a 1" cutoff, you have to cut down the factory pad so much at the toe that you may get all the way down to the steel plate inside the rubber.