"close proximity" is a relative thing, and interpreted. A basic scenario would be a pistol upper, meaning at least an upper receiver with a barrel installed on it, and a rifle.
Push some pins, and you have an illegal sbr in seconds.
However, just a barrel, by itself, and a rifle, is a stretch. Can they get a conviction? Maybe. Tools would be required to disassemble the rifle upper and put the short barrel on it. A short barrel and some more build parts would be better, as it shows you are in the "building process", acquiring parts.
This is where the interpretation comes in. How easy is it to make the illegal item? If you are charged, the judge has to make the same decision.
Push some pins, and you have an illegal sbr in seconds.
However, just a barrel, by itself, and a rifle, is a stretch. Can they get a conviction? Maybe. Tools would be required to disassemble the rifle upper and put the short barrel on it. A short barrel and some more build parts would be better, as it shows you are in the "building process", acquiring parts.
This is where the interpretation comes in. How easy is it to make the illegal item? If you are charged, the judge has to make the same decision.