Hay, 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).

The only problem i see with this is that the AR-15 series is banned by name.  It doesnt say a .223 AR15 or a 9mm AR15 or .22rimfire.  It just says that the AR15 series is banned.  With that said i have seen some AR's that are heavily modified to be "CA" legal so if you can modify it and take the banned featurs off then maybe you can actually build a rimfire one from scratch if it cannot be modified to fire any other caliber.  For that project though i would contact the ATF or whoever you would need to get permission from and get permission in writing from them that you can legally do it.

Sounds good.
The gun will only be an AR-15 on the outside. The inner mechanics will be nothing like the AR-15 but more or less the same as that of the Ruger MARK II and will only be able to fire .22 Magnum and LR ammo and convertable only to .17HMR.
Is that different enough to be "technicley" not an AR-15?

I'm very interested in this. I hope it's approved as legal. Only problem is I'm not an advanced gunsmith so I wouldn't know how to get that done.

It sounds like you would be able to get it ok'd as legal but might be hard to actually get the approval.  You would probably be best off submitting schematics as proof it would only be a rimfire weapon. give it a shot!

Provided they do approve it, how will dealing with the police should I ever have a run in with them be done?

I saw a better and easier idea on homegunsmith


You would leave a portion of the magazine well unfinished/unmilled to act as a block and in turn cut the magazine to fit it, so as a regualr AR mag wouldn't fit.. But I'd leave a larger portion as the block like from the start of the .22 mag to the front and about a inch down..

It would be something best to send away to the  DOJ so others could follow.. if its ok..

Skip the reciever blocks guys...

Just grab a welder and weld a nut on the end of the front takedown pin

Voila! No more 'convertable' gun....

The DPMS and CZ uppers (Bushy wouldn't work 'cause it's n ot metal) both use a .223-only chamber/bolt setup and are 100% incompatible with .223.

So if one were perminantly attached, the gun could never fire other ammo...

So let me get this straight. As long as the rimfire AR-15s receiver is not compatable with AR-15 uppers, the gun is legal in California?

I want to be able to take the gun down for obvious reasons. I have considered the thought of resizing the takedown pin and latch so that a regulear AR-15 upper won't fit provided it's even necessay.
My reason for making the lower the way I have planed goes beyond legalities. I am doing the lower the way I am for additional reasons.

Quick update. I talked to the attourney Generals office and they said it sounds OK but will have to have their expert look at it in the morning and that I can bring it down for inspection and confirmation once it's done if they say it sounds OK.