Did a search found not much people mention it in this section.....

But make sure you "Neuter" (10rd limit and fixed magazine only removable with the aid of a tool)  the upper before attaching to a lower in California!
AR57 is some what impractical to own in Cali.  The whole point of owning this upper is to have access to 50rd of very controllable high velocity round.
But the laws says I can own one if it complies with CA DOJ.  So here it is, built to California DOJ Spec.

http://www.raddlock.com/raddlock_product/ar57_raddlock/AR57.jpg

http://www.raddlock.com/raddlock_product/ar57_raddlock/AR57_Raddlock.jpg

http://www.raddlock.com/raddlock_product/ar57_raddlock/AR57_Raddlock_animate.gif

Radd

This might be another way to go;

There is only one of these but it's gas operated, 18" SS barrel with 1:9 twist and uses the FivenSeven pistol mags. I have a 20 round mag that I use but 10 round mags are available. It shoots the factory 5.7X28 ammo but I prefer to reload to a much hotter level. I don't think this will go any place but the good thing is I can use all standard AR stuff and a nice scope.

Nice custom job.  Who makes your 5.7 AR Bolt and Barrel?

Radd

The barrel is a SS PacNor that was originally 20" HBAR profile and chambered in 5.7X28 but the original owner had the gas port put in the standard rifle location.  I got the barrel for $40 and plugged the original gas port, shortened it to 18", and put in the pistol length gas port. I did the bolt by installing a ring to decrease the bolt face size and then cut out the area for the ejector and modified the extractor a bit.
This is a picture of the bolt face;


There is a bit of other work that must be done to allow the bolt and carrier to slide through the FN pistol magazine feed lips but they are minor compaired to building the bolt. The rifle shoots just like a standard AR except much much cheaper to reload for and it puts the brass in a nice pile at about 5:00 O'clock, about 6 feet behind me. The side cocker handle is from when I first built the rifle and used it first as a straight pull bolt rifle for testing.

Radd, I didn't mean to get into your post I only meant to show there was another way (maybe) to get a California legal 10 round limit with out all the locks and stuff.  I won't go any further.