I have a Double Star California legal lower which I purchase as a receiver only and built up myself. I also have an American Spirit 223 Varmint upper and a TAC SOL 22LR upper. They interchange with the pulling of two captive pins and transferring uppers.
The lower is set up for 223 with the spring buffer in the stock (which the 22LR upper disregards and does not use). As a 223 Varmint gun it is super accurate (Lothar Walther fast twist barrel). The TAC SOL 22LR upper is also very accurate. The gun is very light as a 22LR rifle.
In addition to this I have another AR lower that is used with a 50BMG bolt action upper (BOHICA that actually is 50DTC to be CA legal but essentially the same bullets with slightly modified 50BMG cases). This lower was previously used with the American Spirit upper (223).
The AR is a very versatile rifle with all sorts of caliber uppers available for it. All change over with minimal effort. To get this you have to start with the 223 lower (whether you buy it as a receiver only and build it up or just as a completed lower or as part of a complete 223 rifle). Starting with a 22LR version MAY NOT allow this total interchangeability as some
of the AR 22's are Econo lowers not capable of anything other than 22LR (pot metal parts, etc).
Brownells has a whole catalog of AR parts and pieces. Call them and request their AR catalog. Local gun shows sell all the components and completed guns, even here in Assault Weapons Ban California (a special formula of which parts can be used must be adhered to to be CA legal). There are some dealers around the country that specialize in AR's and other quasi-mIlitary rifles.
For more details on CA legal semi-auto centerfire rifles see Cal Guns web page: http://www.calguns.net/