In the Thompson Center SCOTUS decision that allows handgun/rifle "kits", the Supremes said in regards to an SBR if you have a "group of parts" that can be assembled in both a legal and illegal configuration the government must always assume you only intend to assemble them in a legal manner. Only if you have in fact assembled the parts in an illegal manner can they convict you. "Constructive possession' only applies to an SBR if you have a "group of parts" that can only be assembled in an illegal manner. This is for SBRs only, does not necessarily apply to other NFA items.
Examples: Legal setups, assuming no NFA paperwork:
16" bbl, removable shoulder stock, 10" bbl, pistol gripped receiver that was manufactured originally as a handgun. Pistol/carbine "kit", legal as long as buttstock and short bbl are never installed together. Multiple bbls allowed.
16" bbl rifle, same model in handgun version, interchangeable bbls. Extra bbls in less than 16" length. Legal, spare bbls are for pistol only.
For those with short bbl MGs and a semi full length rifle, the same applies, the bbls legal for the MG are assumed to be for the MG, not the semi rifle.
Examples: Illegal SBR or CP of an SBR:
Less than 16" bbl, rifle receiver with a buttstock attached, no handgun receiver present. Illegal SBR even if bbl is not installed. CP applies, only way to assemble is an SBR.
16" bbl, rifle receiver with buttstock, spare short handgun bbls stored with rifle setup, handgun receiver stored separately. Grey area at best, sure looks like the short bbls were intended for use with the rifle not the handgun. Probably charged and let a Jury sort it out.
Receiver with buttstock attached and short bbl attached. Obviously an SBR regardless of any other circumstances or parts present.
Now this covers fed law, your state law may also apply and that is where your local atty will have to do the research to determine legality.