It's a California thing. SSE stands for Single Shot Exempt. To buy a pistol not on the Ca-DOJ approved handgun list, the pistol has to be made into a single shot pistol, often with a super long barrel. They make single shot magazines for these pistols. After purchasing the SSE pistol, you can modify it to any configuration you want. Every company I'm aware of will buy the SSE parts back so they can reuse them. This is how people have been skirting around the safety requirements. This all comes to an end in Jan-2015. A new law bans any semi auto pistol that has been temporarily disabled to be a single shot.
There are already designs for AR pistols to have no gas tube, and a crooked gas block, making the "pistol" a bolt action... or even selling the pistol without a gas hole in the barrel, and then buying the barrel or upper back.