There is a new tech-branch letter circulating that shotguns that are mfg'd and sold with a pistol grip are now "other" and are not "shotguns" technically, and hence do not come under the SBS 18" barrel limitation of the NFA, as long as they still have a muzzle to grip OAL of 26" or greater.
So in theory, you can now legally have a Rem 870 or Moss 500-590 action with a "witness protection" style grip and a 14" barrel, and it transfers like any other Title I weapon on a mere 4473.
Good luck finding an FFL who'll sell you one without soiling his shorts though.
OTOH, if you're brave, and your state laws are silent on the matter, you can print off the tech-branch letter for yourself, and buy a "cruiser" PG only shotty and cut the barrel yourself, or put an after-market 14" one on.
YMMV, and IANAL etc. And there's been lots of other gun-board drama surrounding this new uncharted territory. (IIRC, someone who's pro-RKBA has even bear-baited the Brady's/VPC with this to try and get these new "others" declared as being DD's, to get some kind of constitutional 2A suit going over this...)
Just FYI.
And of course, the 26" OAL, is still LOTS longer/bigger than a Serbu super-shorty that transfers on an $5 AOW tax. OTOH, if you want to manufacture an AOW yourself and register it (you need a PG'd shotgun from the factory, or a "virgin" receiver that's been neither), it's $200 for us non-SOT people. So in that case, if it's going to cost you $200 anyway, you might as well go SBS, and get a folding stock instead.