We have a fairly restrictive "assault weapon" law here in California. Basically, semi-automatic centerfire rifles that look really scary are banned. Luckily, our M1 Garands, which of course are far more powerful than scary-looking AR-15s and AK-47s, are soft and cuddly looking with those wooden stocks and all, so they aren't subject to the ban. But we can't buy normally configured AR-15s.
For some reason, however, rimfire rifles are not subject to the ban. I guess someone figgerd you can't get killed quite as dead with rimfire rounds as with centerfire rounds (makes as much sense as the rest of the law). So this leaves open lots of opportunities for building up scary looking rifles and carbines that would otherwise be illegal to possess here in the Golden State.
The most popular semi-automatic rimfire is the Ruger 10/22. In its factory furniture, the Ruger 10/22 looks soft and cuddly, which is the way Ruger wants things to remain, generally (all their rifles, even their Min-14 and Mini-30 semi-automatic centerfire rifles, look soft and cuddly, and Ruger refuses to sell larger capacity magazines that project down below the stock, because that would make them look scary; I bet they were WAY embarrassed when Michael Platt chose to use one of their soft and cuddly Mini-14s to shoot five FBI agents in the Miami shootout in 1986).
However, a robust aftermarket has emerged around the 10/22, including stocks and other accoutrements to make them look as scary as you like. Since I have a lot of AR-15 parts laying around the office, I decided to buy a .22 Long Rifle 10/22 and a machined aluminum Rhineland Arms stock to which I could fit an AR-15 buttstock and pistol grip. Then I got a scope and some other goodies to tart it up. This is the result.
Note that it would be a felony for me to set up a centerfire rifle like this in California. But as this is a rimfire, it's all good. Also note that in most of the rest of the US, AR-15s and AK-47s are widely available and pretty popular. Betcha didn't know that.
This stock has generated a bit of buzz over at Rimefire Central.
Late edit: there are ways to legally build ARs and AKs here. I recently built an AR-15 myself.