My 870 SBS had a factory length Wingmaster stock. As did my Waupun Prison Stevens 520 clone. A chop saw and a Miles Gilbert recoil pad installation fixture gave me two shotguns with roughly 12.5" LOP. They are pictured with a 1301 Comp Pro w/ Aimpoint Micro (the easiest to hit with shotgun I have ever fired - 43/50 the first round of Sporting Clays I used it for. Close to a personal record.).
The second pic is the 520 broken down. I plan on making a small case that will hold the shotgun and 15 rounds as a traveling companion.