Just a duplicate of what I have in the legal section. I have plans to make a .22 Magnum ( and later .17HMR ) AR-15 in California.
I have concerns about the legalities. From what I have herd, the problem with DETACHABLE mag AR-15s is the ability to convert them to 5.56 NATO ARs just by dropping in a 5.56 upper with a compatable lower.
What I have in mind is a little different. I do advanced gunsmithing and make major gun parts from scratch. What I have in mind is getting the airsoft gun pictured and described below, stripping out the innerds, removing the barrel and magazine innerds, replacing the barrel with an old .22 barrel, making 10 round housings in the magazines and scratch building a .22 Magnum release-pin action system ( like on the S&W 99 and Colt T-22 ) which then would not be compatable with a 5.56 upper even if you could get it on the gun.
However, I would like to know for sure what I legally can and can't do and am required to do so I won't be breaking any laws in making this.
BTW. If this can be done, then those of you in CA who do advanced gunsmithing may want to consider trying this yourself. The Air Soft comes with all the necessary furnature, and you only have to wory about the barrel and guts, where you can probably get an old .22 rifle barrel for $25, while this Air Soft is only $211 for the whole thing, and the reciever appears to be an inturnally stripped down genuine Colt reciever ( i've handled this air soft rifle before.
If I legally can do this and end up doing it, i'll post an explination of how it was made for you guys to work off of if you like and possably a video of it being fired.

http://img.redwolfairsoft.com/upload/product/img/TM%20M4A1-L.jpg

Tokyo Marui M4A1

One of the most popular AEGs of all time from Marui, the M4A1 sports a EG1000 motor and detachable handle/upper receiver. This is the perfect SOCOM project and is supported by a large aftermarket of accessories including the M500 integrated SureFire light fore-grip. Composite rear stock is extendable. Metal parts include barrel assembly, fully adjustable front/rear sights, magazine, trigger, handle/upper receiver, and T charging handle. Bolt cover flips open when you pull charging handle back just like the real steel! The detachable carrying handle reveals a weaver rail mount that can take a hi-scope mount that allows you to peer over the front sight. A flip up sight is also available. Metal flash hider screws off to reveal counter-clockwise 14mm thread for attachment of silencer (not included).

sounds interesting .... keep us posted...