Any NFA firearm you are building yourself requires ATF Form 1 two copies with passport type photos attatched, a form 5330.20 citizenship form, and two ATF marked fingerprint cards you can order from ATF website, or get from other sources, do not use local cards available from PD and Sheriff.
Fill out the form 1s, attach photos, take to Chief Law Enf. Officer CLEO to sign the forms stating they know no reason why you can not possess a firearm, Mail form 1ns completed fingerprint cards, citizenship form, to NFA branch with a check for $200 for each item registered. Impatiently wait, when form 1 returns approved. Then you can attain and combine with the parts needed to complete your configuration. Durring the time waiting for the form 1 get the receiver or barrel marked with name/city/state in a not hidden spot. You will have to specify a barrel length and overall length and caliber, if you are going to configure differently I would suggest you put in the description text, also to be used with caliber x,y,z barrel lengths x,y,z with and without butt stock overall lengths x,y,z keep all parts needed to configure match with form specs, if changing configuration permanent then later send a notice to the NFA brach, temp changes are covered by the description text.
AOWs and silencers do not need form 5330.20 to transport across state lines, your SBR will before you travel. You can travel through non NFA states like Iowa, etc if your destination state is legal, but do not use in non NFA state. You could use in NON NFA states in NON NFA configuration, by federal law but if your gun is ran by authorities I would expect to loose it when it comes back as registered illegal in that state. If you don't loose the gun bonus for you, but be ready the Chicago cop that wants a new Sawed off Shotgun for duty use just might take it for officer safety. When you sell the gun, either it sells as SBR or you notify the NFA branch to remove gun from registry and then you sell as a title I gun.
If you live somewhere that the CLEO will not sign for you, then you will have to look at Trust or Corporation ownership routes.