18" on shotguns, sans tax stamp
16" on rifles, sans tax stamp
My personal opinion: you can pick up a Remington 870 magnum express (6+1 shots) for around $300 new. This is by far the best deal, IMO for a home defense gun. I'm sure Mossberg makes an equivalent which would work just as well.
Pistol grips are pretty pointless, IMO, on a shotgun such as this. They might look cool, but for a tube-fed shotgun, I'd think they'd just make reloads slower. And they certainly don't have any advantage over a regular grip.
Edit* - just saw the one you were looking at. Not trying to be a dick, but, that thing's a joke. That brake is pointless, and heavy. The barrel shroud (the shoulder thing that goes up) is also pointless, and just adds weight to the gun. And, you really should have a shoulder stock on a gun like this. It'll give you way more control. Ever tried to aim a shotgun without a stock? Those guns look neat, but they aren't practical, IMO.