What you are refering to is "Constructive Possesion". As far as all of my research shows that the only "CP" is with a machine gun.
ALA:
http://www.cs.cmu.edu/afs/cs/usr/wbardwel/public/nfalist/atf_letter7.txt
and:
M. FIREARMS - NATIONAL FIREARMS ACT (NFA)
(M1) The types of firearms that must be registered in the National Firearm Registration and Transfer Record are defined in the NFA and in 27 CFR Part 179. What are some examples? [Back]
Some examples of the types of firearms that must be registered are:
Machineguns;
The frames or receivers of machineguns;
Any combination of parts designed and intended for use in converting weapons into machineguns;
Any part designed and intended solely and exclusively for converting a weapon into a machinegun;
Any combination of parts from which a machinegun can be assembled if the parts are in the possession or under the control of a person;
Silencers and any part designed and intended for fabricating a silencer;
Short-barreled rifles;
Short-barreled shotguns;
Destructive devices; and,
"Any other weapons."
A few examples of destructive devices are:
Molotov cocktails;
Anti-tank guns (over caliber .50);
Bazookas; and,
Mortars.
A few examples of "any other weapon" are:
H& R Handyguns;
Ithaca Auto-Burglar guns;
Cane guns; and,
Gadget-type firearms and "pen" guns which fire fixed ammunition.
Nothing in there says "Parts to convert a gun into an SBR", Yet there are specific provisions for Machine guns.......