What's involved in selling a SBR?

Form 4 to a private individual in your state or to an NFA dealer out of state.  Or return it to Title I status and sell it as such with no paperwork or wait.

 

So, I can return the weapon to a 16'' barrel and just sell it as if it wasn't a SBR?
Would I have to contact the ATF to tell them?

Yes and yes.

Thanks for all the help.
Last question, I contact the ATF before or after I sell it?

Before!! Otherwise you can be in some deep crap.

thanks man.

From the ATF SBR/SBS FAQ website (http://www.atf.gov/firearms/faq/national-firearms-act-short-barreled-rifles-shotguns.html):

Q: I possess a properly registered SBR or SBS. I intend to strip the receiver and remove the barrel prior to selling the receiver. Is the bare receiver still subject to regulation under the NFA as a SBR or SBS?
A stripped receiver without a barrel does not meet the definition of a SBR or SBS under the NFA. Although the previously registered firearm would remain registered unless the possessor notified the NFA Branch of the change, there is no provision in statute or regulation requiring registration of a firearm without a barrel because its physical characteristics would make it only a GCA “firearm” pursuant to 18 U.S.C. § 921(a)(3)(B). If the subsequent owner buys the receiver as a GCA firearm and installs a barrel less than 16 inches in length (SBR) or 18 inches in length (SBS), the firearm would be subject to a $200 making tax and registration under the NFA by the manufacturer or maker of the SBR or SBS. Because registration depends upon the stated intent of the applicant, there is no provision to allow registration of a NFA firearm by anyone other than the maker or manufacturer.

Q: May I transfer the receiver of a short-barrel rifle or shotgun to an FFL or to an individual as I would any GCA firearm?
Yes. A weapon that does not meet the definition of a NFA “firearm” is not subject to the NFA and a possessor or transferor needn’t comply with NFA requirements. The firearm is considered a GCA firearm and may be transferred under the provisions of that law.

Q: Is it necessary to send notification to ATF and receive acknowledgement that the SBR or SBS has been removed from the purview of the NFA before it may be sold as a GCA firearm?
There is no requirement for the possessor of a registered NFA firearm to notify ATF that the firearm has been removed from the purview of the NFA. However, ATF recommends the possessor notify the NFA Branch of such changes in writing so that the possessor is not mistakenly identified as the owner if the firearm is later used in a crime. If, at the time of transfer, the firearm does not meet the definition of a SBR, it should be transferred without filing the NFA transfer application and without payment of the transfer tax.