If I puchase a registered auto sear and place it in my AR15, with the required M16 parts would it be legal? I thought it would be illegal to put M16 parts in a civ AR15. but as we all know adding a autosear will not make the rifle full auto. you have to add the M16 parts for it to function, and that would be illegal considering the receiver is not a NFA receiver, right? any advice would be appreciated thanks, Joe

Once you add the RDIAS it's considered a Title II firearm and it's no longer illegal, as you stated, you need the full-auto bolt, etc for it to function properly. If the RDIAS is removed from the receiver, then you must also remove the other full-auto accessories.

If you had a "real" registered auto sear you could add it and whatever parts were needed to convert the rifle to full auto. That's the point of having the sear. By "real" I mean not one of those DIAS that used to be advertised in ShotGun News or elsewhere for $250.

As a side benifit if you added the RDIAS to a Post Ban AR-15 you could then add whatever features you wanted to add that are restricted by the '94 Assault Weapons Ban. Such as a bayonet lug, flash suppressor, folding stock, etc. But once you remove the sear you have to remove the evil parts also.

The RDIAS is what's considered the machinegun.  The AR is nothing but a piece of metal in which you insert the RDIAS.  Once you "marry" the DIAS to the AR, then you can legally use M16 parts.

"Real" also meaning you obtained the RDIAS through a Form 4.  Also, remember that you can't put your RDIAS in anything unless it is done through a Form 1.  If you do, it can be married to anything.  Once the RDIAS is divorced from the lower, then the Crime Bill again applies.

Come again, Steve?  Form 1 is neither required nor allowed to "marry" a DIAS.  That is accomplished by sending in a letter to ATF asking that the serial number of the host gun be added to the 'description' section of the Form 4.

Divorce is accomplished by sending in a letter to the ATF asking that the serial number of the host gun be removed from 'description' section of the Form 4.

Form 1 is only for the tax paid ($200) making of a new NFA firearm.

An unmarried DIAS can be freely put into ANY AR-15, and while installed that gun is a machinegun and is not subject to the AW ban restrictions.  AW features and full auto parts should not be installed unless the DIAS is installed, and must come off (AW features can stay on a pre-ban) when the DIAS is not installed.

What?

I thought that if you bought a RDIAS per a form 4 and you want to "marry" it to, say, a post pan Bushy lower, you had to do it per a form 1.  Have I been mistaken?  

Never owned one but I was sure that was the procedure.

Splain yourself.

Circuits is right, ignore my above reply.  I can't believe no one has called me on it before since I've said it more than once. [chainsaw]

Follow up:  Do you even need the letter.

Strictly speaking, no - although ATF prefers that you keep your location and identification paperwork up to date.  If there's no host gun serial number on the Form 4, the DIAS is "single", and free to date - although even a "married" sear can go out with other guns on the side. :)