[Msig47]

Can a registered mpx sbr be converted to full auto by sig? If so, what would be the legal procedure?

 

[InOverMyHead]

Yes, because they have a license to do so (they are legally licensed to create new full auto weapons for "demonstration" and to sell to permitted agencies.).
They can't sell it to am agency not licensed and it can't be personally owned.
Thus your SBR can not be converted and you retain ownership.

I think I got that correct.

 

[COBE]

The 1986 Firearm Owners' Protection Act barred any newly produced fully automatic weapon from possession by civilians. You can still own machine guns manufactured prior to 86 (at a hefty price).

 

[wetwrks]

Unless you are a dealer or manufacturer...