It is
POSSIBLE to shorten the LOP on a Mossberg 930 stock.
The problem is, the recoil spring and tube that run down the length of the stock.
Here is a pictire of the nut that holds the stock on the tube....you can see there is almost 1/4" of threads exposed on the recoil tube.
The total length of the recoil tube is 11-9/16"
The spacer is roughly 5/8" wide
The nut itself is 7/16" wide
Cut the nut in half (or more), replace the spacer with a washer, remove the excess threads on the recoil tube (~7/16").... This is all without changing the spring length or anything "functional".
Place the synthetic stock back on and measure the distance from the outer edge of the stock to the end of the recoil tube. Should be about 1-1/4" or so.
Set up the table saw to that measurement and cut the stock back by whatever dimension you got above...
Get a Limbsaver "grind to fit" recoil pad, they're slightly thinner that the stock pad from Mossberg...
All of the above should take the "factory" LOP from 14" down to 12-1/2" or so...
It might be better to use a wood stock, too... They can be bought from Mossberg directly...