Does anyone know how to shorten the synthetic stock or where I can buy a shorter one?

Just out of curiosity, how tall are you?  I'd like a Benelli M1 and their website say the length of pull is 14 3/8 inches!  That does seem long.

I'm 5'10 and its way too long, especially with body armor.

I don't know of anyone making a shorter stock as of yet. The problem with doing that is making a shorter recoil tube assembly. Shortening the tube will also involve playing with the cyclic rate of the gun and all kinds of other stuff beyond me. Bronwnells offers a recoil pad adapter that allows the placement of a aftermarket recoil pad on a benelli. A guy could look at using the adapter only and that would drop off about 1" from the length of pull. Sounds like you are using the gun in a SAS stance. Be patient....I know of one manufacturer that WILL have a short AR15/M16 stock  for this purpose along with an 870 short stock for starters.These will be availible soon. I have already begun the discussion of a short benelli stock with them. Watch the manufacturer forums and shot show info over the next few weeks for info.

Hiredgun
Thank you for the information.  Im eager to learn more.  If you dont mind send me the companies contact information so I can bug them also.

I'm 5'5", and I had the same problem, only worse.  I couldn't bring the shotgun up quickly enought without snagging under my armpit.  Never did find a solution, though I considered removing the plate and grinding down the butstock about 1".  Finally, I just sold the thing.

im 6'1'' and it fits me just fine