Unlike TV shows, very short shotguns kick like heck! I mean kick badly, like a .44 magnum derringer. And my Mossie 500 has a short stock I made myself. Just got the regular stock, used a band saw and level as a guide, and cut it down a few inches. I then buffered the recoil pad to match and it works perfect. Just keep you nose away from your shooting hands thumb or it will smack your nose.
Makes my Mossie only one inch longer than my M1 Carbine, and it still has a usable stock.
I believe though you will have to contact the ATF directly for any how-to advice. To do this by internet guidence is to end up telling the AFT, after they arrest you, that you thought it was legal.